workqueue: Backport from upstream

workqueue: change BUG_ON() to WARN_ON()

This BUG_ON() can be triggered if you call schedule_work() before
calling INIT_WORK().  It is a bug definitely, but it's nicer to just
print a stack trace and return.

Reported-by: Matt Renzelmann <mjr@cs.wisc.edu>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: Catch more locking problems with flush_work()

If a workqueue is flushed with flush_work() lockdep checking can
be circumvented. For example:

 static DEFINE_MUTEX(mutex);

 static void my_work(struct work_struct *w)
 {
         mutex_lock(&mutex);
         mutex_unlock(&mutex);
 }

 static DECLARE_WORK(work, my_work);

 static int __init start_test_module(void)
 {
         schedule_work(&work);
         return 0;
 }
 module_init(start_test_module);

 static void __exit stop_test_module(void)
 {
         mutex_lock(&mutex);
         flush_work(&work);
         mutex_unlock(&mutex);
 }
 module_exit(stop_test_module);

would not always print a warning when flush_work() was called.
In this trivial example nothing could go wrong since we are
guaranteed module_init() and module_exit() don't run concurrently,
but if the work item is schedule asynchronously we could have a
scenario where the work item is running just at the time flush_work()
is called resulting in a classic ABBA locking problem.

Add a lockdep hint by acquiring and releasing the work item
lockdep_map in flush_work() so that we always catch this
potential deadlock scenario.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Reviewed-by: Yong Zhang <yong.zhang0@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

lockdep: fix oops in processing workqueue

Under memory load, on x86_64, with lockdep enabled, the workqueue's
process_one_work() has been seen to oops in __lock_acquire(), barfing
on a 0xffffffff00000000 pointer in the lockdep_map's class_cache[].

Because it's permissible to free a work_struct from its callout function,
the map used is an onstack copy of the map given in the work_struct: and
that copy is made without any locking.

Surprisingly, gcc (4.5.1 in Hugh's case) uses "rep movsl" rather than
"rep movsq" for that structure copy: which might race with a workqueue
user's wait_on_work() doing lock_map_acquire() on the source of the
copy, putting a pointer into the class_cache[], but only in time for
the top half of that pointer to be copied to the destination map.

Boom when process_one_work() subsequently does lock_map_acquire()
on its onstack copy of the lockdep_map.

Fix this, and a similar instance in call_timer_fn(), with a
lockdep_copy_map() function which additionally NULLs the class_cache[].

Note: this oops was actually seen on 3.4-next, where flush_work() newly
does the racing lock_map_acquire(); but Tejun points out that 3.4 and
earlier are already vulnerable to the same through wait_on_work().

* Patch orginally from Peter.  Hugh modified it a bit and wrote the
  description.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Reported-by: Hugh Dickins <hughd@google.com>
LKML-Reference: <alpine.LSU.2.00.1205070951170.1544@eggly.anvils>
Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: perform cpu down operations from low priority cpu_notifier()

Currently, all workqueue cpu hotplug operations run off
CPU_PRI_WORKQUEUE which is higher than normal notifiers.  This is to
ensure that workqueue is up and running while bringing up a CPU before
other notifiers try to use workqueue on the CPU.

Per-cpu workqueues are supposed to remain working and bound to the CPU
for normal CPU_DOWN_PREPARE notifiers.  This holds mostly true even
with workqueue offlining running with higher priority because
workqueue CPU_DOWN_PREPARE only creates a bound trustee thread which
runs the per-cpu workqueue without concurrency management without
explicitly detaching the existing workers.

However, if the trustee needs to create new workers, it creates
unbound workers which may wander off to other CPUs while
CPU_DOWN_PREPARE notifiers are in progress.  Furthermore, if the CPU
down is cancelled, the per-CPU workqueue may end up with workers which
aren't bound to the CPU.

While reliably reproducible with a convoluted artificial test-case
involving scheduling and flushing CPU burning work items from CPU down
notifiers, this isn't very likely to happen in the wild, and, even
when it happens, the effects are likely to be hidden by the following
successful CPU down.

Fix it by using different priorities for up and down notifiers - high
priority for up operations and low priority for down operations.

Workqueue cpu hotplug operations will soon go through further cleanup.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>

workqueue: drop CPU_DYING notifier operation

Workqueue used CPU_DYING notification to mark GCWQ_DISASSOCIATED.
This was necessary because workqueue's CPU_DOWN_PREPARE happened
before other DOWN_PREPARE notifiers and workqueue needed to stay
associated across the rest of DOWN_PREPARE.

After the previous patch, workqueue's DOWN_PREPARE happens after
others and can set GCWQ_DISASSOCIATED directly.  Drop CPU_DYING and
let the trustee set GCWQ_DISASSOCIATED after disabling concurrency
management.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>

workqueue: ROGUE workers are UNBOUND workers

Currently, WORKER_UNBOUND is used to mark workers for the unbound
global_cwq and WORKER_ROGUE is used to mark workers for disassociated
per-cpu global_cwqs.  Both are used to make the marked worker skip
concurrency management and the only place they make any difference is
in worker_enter_idle() where WORKER_ROGUE is used to skip scheduling
idle timer, which can easily be replaced with trustee state testing.

This patch replaces WORKER_ROGUE with WORKER_UNBOUND and drops
WORKER_ROGUE.  This is to prepare for removing trustee and handling
disassociated global_cwqs as unbound.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>

workqueue: use mutex for global_cwq manager exclusion

POOL_MANAGING_WORKERS is used to ensure that at most one worker takes
the manager role at any given time on a given global_cwq.  Trustee
later hitched on it to assume manager adding blocking wait for the
bit.  As trustee already needed a custom wait mechanism, waiting for
MANAGING_WORKERS was rolled into the same mechanism.

Trustee is scheduled to be removed.  This patch separates out
MANAGING_WORKERS wait into per-pool mutex.  Workers use
mutex_trylock() to test for manager role and trustee uses mutex_lock()
to claim manager roles.

gcwq_claim/release_management() helpers are added to grab and release
manager roles of all pools on a global_cwq.  gcwq_claim_management()
always grabs pool manager mutexes in ascending pool index order and
uses pool index as lockdep subclass.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>

workqueue: drop @bind from create_worker()

Currently, create_worker()'s callers are responsible for deciding
whether the newly created worker should be bound to the associated CPU
and create_worker() sets WORKER_UNBOUND only for the workers for the
unbound global_cwq.  Creation during normal operation is always via
maybe_create_worker() and @bind is true.  For workers created during
hotplug, @bind is false.

Normal operation path is planned to be used even while the CPU is
going through hotplug operations or offline and this static decision
won't work.

Drop @bind from create_worker() and decide whether to bind by looking
at GCWQ_DISASSOCIATED.  create_worker() will also set WORKER_UNBOUND
autmatically if disassociated.  To avoid flipping GCWQ_DISASSOCIATED
while create_worker() is in progress, the flag is now allowed to be
changed only while holding all manager_mutexes on the global_cwq.

This requires that GCWQ_DISASSOCIATED is not cleared behind trustee's
back.  CPU_ONLINE no longer clears DISASSOCIATED before flushing
trustee, which clears DISASSOCIATED before rebinding remaining workers
if asked to release.  For cases where trustee isn't around, CPU_ONLINE
clears DISASSOCIATED after flushing trustee.  Also, now, first_idle
has UNBOUND set on creation which is explicitly cleared by CPU_ONLINE
while binding it.  These convolutions will soon be removed by further
simplification of CPU hotplug path.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>

workqueue: reimplement CPU online rebinding to handle idle workers

Currently, if there are left workers when a CPU is being brough back
online, the trustee kills all idle workers and scheduled rebind_work
so that they re-bind to the CPU after the currently executing work is
finished.  This works for busy workers because concurrency management
doesn't try to wake up them from scheduler callbacks, which require
the target task to be on the local run queue.  The busy worker bumps
concurrency counter appropriately as it clears WORKER_UNBOUND from the
rebind work item and it's bound to the CPU before returning to the
idle state.

To reduce CPU on/offlining overhead (as many embedded systems use it
for powersaving) and simplify the code path, workqueue is planned to
be modified to retain idle workers across CPU on/offlining.  This
patch reimplements CPU online rebinding such that it can also handle
idle workers.

As noted earlier, due to the local wakeup requirement, rebinding idle
workers is tricky.  All idle workers must be re-bound before scheduler
callbacks are enabled.  This is achieved by interlocking idle
re-binding.  Idle workers are requested to re-bind and then hold until
all idle re-binding is complete so that no bound worker starts
executing work item.  Only after all idle workers are re-bound and
parked, CPU_ONLINE proceeds to release them and queue rebind work item
to busy workers thus guaranteeing scheduler callbacks aren't invoked
until all idle workers are ready.

worker_rebind_fn() is renamed to busy_worker_rebind_fn() and
idle_worker_rebind() for idle workers is added.  Rebinding logic is
moved to rebind_workers() and now called from CPU_ONLINE after
flushing trustee.  While at it, add CPU sanity check in
worker_thread().

Note that now a worker may become idle or the manager between trustee
release and rebinding during CPU_ONLINE.  As the previous patch
updated create_worker() so that it can be used by regular manager
while unbound and this patch implements idle re-binding, this is safe.

This prepares for removal of trustee and keeping idle workers across
CPU hotplugs.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>

workqueue: don't butcher idle workers on an offline CPU

Currently, during CPU offlining, after all pending work items are
drained, the trustee butchers all workers.  Also, on CPU onlining
failure, workqueue_cpu_callback() ensures that the first idle worker
is destroyed.  Combined, these guarantee that an offline CPU doesn't
have any worker for it once all the lingering work items are finished.

This guarantee isn't really necessary and makes CPU on/offlining more
expensive than needs to be, especially for platforms which use CPU
hotplug for powersaving.

This patch lets offline CPUs removes idle worker butchering from the
trustee and let a CPU which failed onlining keep the created first
worker.  The first worker is created if the CPU doesn't have any
during CPU_DOWN_PREPARE and started right away.  If onlining succeeds,
the rebind_workers() call in CPU_ONLINE will rebind it like any other
workers.  If onlining fails, the worker is left alone till the next
try.

This makes CPU hotplugs cheaper by allowing global_cwqs to keep
workers across them and simplifies code.

Note that trustee doesn't re-arm idle timer when it's done and thus
the disassociated global_cwq will keep all workers until it comes back
online.  This will be improved by further patches.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>

workqueue: remove CPU offline trustee

With the previous changes, a disassociated global_cwq now can run as
an unbound one on its own - it can create workers as necessary to
drain remaining works after the CPU has been brought down and manage
the number of workers using the usual idle timer mechanism making
trustee completely redundant except for the actual unbinding
operation.

This patch removes the trustee and let a disassociated global_cwq
manage itself.  Unbinding is moved to a work item (for CPU affinity)
which is scheduled and flushed from CPU_DONW_PREPARE.

This patch moves nr_running clearing outside gcwq and manager locks to
simplify the code.  As nr_running is unused at the point, this is
safe.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>

workqueue: simplify CPU hotplug code

With trustee gone, CPU hotplug code can be simplified.

* gcwq_claim/release_management() now grab and release gcwq lock too
  respectively and gained _and_lock and _and_unlock postfixes.

* All CPU hotplug logic was implemented in workqueue_cpu_callback()
  which was called by workqueue_cpu_up/down_callback() for the correct
  priority.  This was because up and down paths shared a lot of logic,
  which is no longer true.  Remove workqueue_cpu_callback() and move
  all hotplug logic into the two actual callbacks.

This patch doesn't make any functional changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>

workqueue: fix spurious CPU locality WARN from process_one_work()

25511a4776 "workqueue: reimplement CPU online rebinding to handle idle
workers" added CPU locality sanity check in process_one_work().  It
triggers if a worker is executing on a different CPU without UNBOUND
or REBIND set.

This works for all normal workers but rescuers can trigger this
spuriously when they're serving the unbound or a disassociated
global_cwq - rescuers don't have either flag set and thus its
gcwq->cpu can be a different value including %WORK_CPU_UNBOUND.

Fix it by additionally testing %GCWQ_DISASSOCIATED.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
LKML-Refence: <20120721213656.GA7783@linux.vnet.ibm.com>

workqueue: reorder queueing functions so that _on() variants are on top

Currently, queue/schedule[_delayed]_work_on() are located below the
counterpart without the _on postifx even though the latter is usually
implemented using the former.  Swap them.

This is cleanup and doesn't cause any functional difference.

Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: make queueing functions return bool

All queueing functions return 1 on success, 0 if the work item was
already pending.  Update them to return bool instead.  This signifies
better that they don't return 0 / -errno.

This is cleanup and doesn't cause any functional difference.

While at it, fix comment opening for schedule_work_on().

Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: add missing smp_wmb() in process_one_work()

WORK_STRUCT_PENDING is used to claim ownership of a work item and
process_one_work() releases it before starting execution.  When
someone else grabs PENDING, all pre-release updates to the work item
should be visible and all updates made by the new owner should happen
afterwards.

Grabbing PENDING uses test_and_set_bit() and thus has a full barrier;
however, clearing doesn't have a matching wmb.  Given the preceding
spin_unlock and use of clear_bit, I don't believe this can be a
problem on an actual machine and there hasn't been any related report
but it still is theretically possible for clear_pending to permeate
upwards and happen before work->entry update.

Add an explicit smp_wmb() before work_clear_pending().

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: stable@vger.kernel.org

workqueue: disable irq while manipulating PENDING

Queueing operations use WORK_STRUCT_PENDING_BIT to synchronize access
to the target work item.  They first try to claim the bit and proceed
with queueing only after that succeeds and there's a window between
PENDING being set and the actual queueing where the task can be
interrupted or preempted.

There's also a similar window in process_one_work() when clearing
PENDING.  A work item is dequeued, gcwq->lock is released and then
PENDING is cleared and the worker might get interrupted or preempted
between releasing gcwq->lock and clearing PENDING.

cancel[_delayed]_work_sync() tries to claim or steal PENDING.  The
function assumes that a work item with PENDING is either queued or in
the process of being [de]queued.  In the latter case, it busy-loops
until either the work item loses PENDING or is queued.  If canceling
coincides with the above described interrupts or preemptions, the
canceling task will busy-loop while the queueing or executing task is
preempted.

This patch keeps irq disabled across claiming PENDING and actual
queueing and moves PENDING clearing in process_one_work() inside
gcwq->lock so that busy looping from PENDING && !queued doesn't wait
for interrupted/preempted tasks.  Note that, in process_one_work(),
setting last CPU and clearing PENDING got merged into single
operation.

This removes possible long busy-loops and will allow using
try_to_grab_pending() from bh and irq contexts.

v2: __queue_work() was testing preempt_count() to ensure that the
    caller has disabled preemption.  This triggers spuriously if
    !CONFIG_PREEMPT_COUNT.  Use preemptible() instead.  Reported by
    Fengguang Wu.

v3: Disable irq instead of preemption.  IRQ will be disabled while
    grabbing gcwq->lock later anyway and this allows using
    try_to_grab_pending() from bh and irq contexts.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Fengguang Wu <fengguang.wu@intel.com>

workqueue: set delayed_work->timer function on initialization

delayed_work->timer.function is currently initialized during
queue_delayed_work_on().  Export delayed_work_timer_fn() and set
delayed_work timer function during delayed_work initialization
together with other fields.

This ensures the timer function is always valid on an initialized
delayed_work.  This is to help mod_delayed_work() implementation.

To detect delayed_work users which diddle with the internal timer,
trigger WARN if timer function doesn't match on queue.

Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: unify local CPU queueing handling

Queueing functions have been using different methods to determine the
local CPU.

* queue_work() superflously uses get/put_cpu() to acquire and hold the
  local CPU across queue_work_on().

* delayed_work_timer_fn() uses smp_processor_id().

* queue_delayed_work() calls queue_delayed_work_on() with -1 @cpu
  which is interpreted as the local CPU.

* flush_delayed_work[_sync]() were using raw_smp_processor_id().

* __queue_work() interprets %WORK_CPU_UNBOUND as local CPU if the
  target workqueue is bound one but nobody uses this.

This patch converts all functions to uniformly use %WORK_CPU_UNBOUND
to indicate local CPU and use the local binding feature of
__queue_work().  unlikely() is dropped from %WORK_CPU_UNBOUND handling
in __queue_work().

Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: fix zero @delay handling of queue_delayed_work_on()

If @delay is zero and the dealyed_work is idle, queue_delayed_work()
queues it for immediate execution; however, queue_delayed_work_on()
lacks this logic and always goes through timer regardless of @delay.

This patch moves 0 @delay handling logic from queue_delayed_work() to
queue_delayed_work_on() so that both functions behave the same.

Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: move try_to_grab_pending() upwards

try_to_grab_pending() will be used by to-be-implemented
mod_delayed_work[_on]().  Move try_to_grab_pending() and related
functions above queueing functions.

This patch only moves functions around.

Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: introduce WORK_OFFQ_FLAG_*

Low WORK_STRUCT_FLAG_BITS bits of work_struct->data contain
WORK_STRUCT_FLAG_* and flush color.  If the work item is queued, the
rest point to the cpu_workqueue with WORK_STRUCT_CWQ set; otherwise,
WORK_STRUCT_CWQ is clear and the bits contain the last CPU number -
either a real CPU number or one of WORK_CPU_*.

Scheduled addition of mod_delayed_work[_on]() requires an additional
flag, which is used only while a work item is off queue.  There are
more than enough bits to represent off-queue CPU number on both 32 and
64bits.  This patch introduces WORK_OFFQ_FLAG_* which occupy the lower
part of the @work->data high bits while off queue.  This patch doesn't
define any actual OFFQ flag yet.

Off-queue CPU number is now shifted by WORK_OFFQ_CPU_SHIFT, which adds
the number of bits used by OFFQ flags to WORK_STRUCT_FLAG_SHIFT, to
make room for OFFQ flags.

To avoid shift width warning with large WORK_OFFQ_FLAG_BITS, ulong
cast is added to WORK_STRUCT_NO_CPU and, just in case, BUILD_BUG_ON()
to check that there are enough bits to accomodate off-queue CPU number
is added.

This patch doesn't make any functional difference.

Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: factor out __queue_delayed_work() from queue_delayed_work_on()

This is to prepare for mod_delayed_work[_on]() and doesn't cause any
functional difference.

Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: reorganize try_to_grab_pending() and __cancel_timer_work()

* Use bool @is_dwork instead of @timer and let try_to_grab_pending()
  use to_delayed_work() to determine the delayed_work address.

* Move timer handling from __cancel_work_timer() to
  try_to_grab_pending().

* Make try_to_grab_pending() use -EAGAIN instead of -1 for
  busy-looping and drop the ret local variable.

* Add proper function comment to try_to_grab_pending().

This makes the code a bit easier to understand and will ease further
changes.  This patch doesn't make any functional change.

v2: Use @is_dwork instead of @timer.

Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: mark a work item being canceled as such

There can be two reasons try_to_grab_pending() can fail with -EAGAIN.
One is when someone else is queueing or deqeueing the work item.  With
the previous patches, it is guaranteed that PENDING and queued state
will soon agree making it safe to busy-retry in this case.

The other is if multiple __cancel_work_timer() invocations are racing
one another.  __cancel_work_timer() grabs PENDING and then waits for
running instances of the target work item on all CPUs while holding
PENDING and !queued.  try_to_grab_pending() invoked from another task
will keep returning -EAGAIN while the current owner is waiting.

Not distinguishing the two cases is okay because __cancel_work_timer()
is the only user of try_to_grab_pending() and it invokes
wait_on_work() whenever grabbing fails.  For the first case, busy
looping should be fine but wait_on_work() doesn't cause any critical
problem.  For the latter case, the new contender usually waits for the
same condition as the current owner, so no unnecessarily extended
busy-looping happens.  Combined, these make __cancel_work_timer()
technically correct even without irq protection while grabbing PENDING
or distinguishing the two different cases.

While the current code is technically correct, not distinguishing the
two cases makes it difficult to use try_to_grab_pending() for other
purposes than canceling because it's impossible to tell whether it's
safe to busy-retry grabbing.

This patch adds a mechanism to mark a work item being canceled.
try_to_grab_pending() now disables irq on success and returns -EAGAIN
to indicate that grabbing failed but PENDING and queued states are
gonna agree soon and it's safe to busy-loop.  It returns -ENOENT if
the work item is being canceled and it may stay PENDING && !queued for
arbitrary amount of time.

__cancel_work_timer() is modified to mark the work canceling with
WORK_OFFQ_CANCELING after grabbing PENDING, thus making
try_to_grab_pending() fail with -ENOENT instead of -EAGAIN.  Also, it
invokes wait_on_work() iff grabbing failed with -ENOENT.  This isn't
necessary for correctness but makes it consistent with other future
users of try_to_grab_pending().

v2: try_to_grab_pending() was testing preempt_count() to ensure that
    the caller has disabled preemption.  This triggers spuriously if
    !CONFIG_PREEMPT_COUNT.  Use preemptible() instead.  Reported by
    Fengguang Wu.

v3: Updated so that try_to_grab_pending() disables irq on success
    rather than requiring preemption disabled by the caller.  This
    makes busy-looping easier and will allow try_to_grap_pending() to
    be used from bh/irq contexts.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Fengguang Wu <fengguang.wu@intel.com>

workqueue: implement mod_delayed_work[_on]()

Workqueue was lacking a mechanism to modify the timeout of an already
pending delayed_work.  delayed_work users have been working around
this using several methods - using an explicit timer + work item,
messing directly with delayed_work->timer, and canceling before
re-queueing, all of which are error-prone and/or ugly.

This patch implements mod_delayed_work[_on]() which behaves similarly
to mod_timer() - if the delayed_work is idle, it's queued with the
given delay; otherwise, its timeout is modified to the new value.
Zero @delay guarantees immediate execution.

v2: Updated to reflect try_to_grab_pending() changes.  Now safe to be
    called from bh context.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Ingo Molnar <mingo@redhat.com>

workqueue: fix CPU binding of flush_delayed_work[_sync]()

delayed_work encodes the workqueue to use and the last CPU in
delayed_work->work.data while it's on timer.  The target CPU is
implicitly recorded as the CPU the timer is queued on and
delayed_work_timer_fn() queues delayed_work->work to the CPU it is
running on.

Unfortunately, this leaves flush_delayed_work[_sync]() no way to find
out which CPU the delayed_work was queued for when they try to
re-queue after killing the timer.  Currently, it chooses the local CPU
flush is running on.  This can unexpectedly move a delayed_work queued
on a specific CPU to another CPU and lead to subtle errors.

There isn't much point in trying to save several bytes in struct
delayed_work, which is already close to a hundred bytes on 64bit with
all debug options turned off.  This patch adds delayed_work->cpu to
remember the CPU it's queued for.

Note that if the timer is migrated during CPU down, the work item
could be queued to the downed global_cwq after this change.  As a
detached global_cwq behaves like an unbound one, this doesn't change
much for the delayed_work.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>

workqueue: add missing wmb() in clear_work_data()

Any operation which clears PENDING should be preceded by a wmb to
guarantee that the next PENDING owner sees all the changes made before
PENDING release.

There are only two places where PENDING is cleared -
set_work_cpu_and_clear_pending() and clear_work_data().  The caller of
the former already does smp_wmb() but the latter doesn't have any.

Move the wmb above set_work_cpu_and_clear_pending() into it and add
one to clear_work_data().

There hasn't been any report related to this issue, and, given how
clear_work_data() is used, it is extremely unlikely to have caused any
actual problems on any architecture.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>

workqueue: use enum value to set array size of pools in gcwq

Commit 3270476a6c0ce322354df8679652f060d66526dc ('workqueue: reimplement
WQ_HIGHPRI using a separate worker_pool') introduce separate worker_pool
for HIGHPRI. Although there is NR_WORKER_POOLS enum value which represent
size of pools, definition of worker_pool in gcwq doesn't use it.
Using it makes code robust and prevent future mistakes.
So change code to use this enum value.

Signed-off-by: Joonsoo Kim <js1304@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: correct req_cpu in trace_workqueue_queue_work()

When we do tracing workqueue_queue_work(), it records requested cpu.
But, if !(@wq->flag & WQ_UNBOUND) and @cpu is WORK_CPU_UNBOUND,
requested cpu is changed as local cpu.
In case of @wq->flag & WQ_UNBOUND, above change is not occured,
therefore it is reasonable to correct it.

Use temporary local variable for storing requested cpu.

Signed-off-by: Joonsoo Kim <js1304@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: change value of lcpu in __queue_delayed_work_on()

We assign cpu id into work struct's data field in __queue_delayed_work_on().
In current implementation, when work is come in first time,
current running cpu id is assigned.
If we do __queue_delayed_work_on() with CPU A on CPU B,
__queue_work() invoked in delayed_work_timer_fn() go into
the following sub-optimal path in case of WQ_NON_REENTRANT.

	gcwq = get_gcwq(cpu);
	if (wq->flags & WQ_NON_REENTRANT &&
		(last_gcwq = get_work_gcwq(work)) && last_gcwq != gcwq) {

Change lcpu to @cpu and rechange lcpu to local cpu if lcpu is WORK_CPU_UNBOUND.
It is sufficient to prevent to go into sub-optimal path.

tj: Slightly rephrased the comment.

Signed-off-by: Joonsoo Kim <js1304@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: introduce system_highpri_wq

Commit 3270476a6c0ce322354df8679652f060d66526dc ('workqueue: reimplement
WQ_HIGHPRI using a separate worker_pool') introduce separate worker pool
for HIGHPRI. When we handle busyworkers for gcwq, it can be normal worker
or highpri worker. But, we don't consider this difference in rebind_workers(),
we use just system_wq for highpri worker. It makes mismatch between
cwq->pool and worker->pool.

It doesn't make error in current implementation, but possible in the future.
Now, we introduce system_highpri_wq to use proper cwq for highpri workers
in rebind_workers(). Following patch fix this issue properly.

tj: Even apart from rebinding, having system_highpri_wq generally
    makes sense.

Signed-off-by: Joonsoo Kim <js1304@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: use system_highpri_wq for highpri workers in rebind_workers()

In rebind_workers(), we do inserting a work to rebind to cpu for busy workers.
Currently, in this case, we use only system_wq. This makes a possible
error situation as there is mismatch between cwq->pool and worker->pool.

To prevent this, we should use system_highpri_wq for highpri worker
to match theses. This implements it.

tj: Rephrased comment a bit.

Signed-off-by: Joonsoo Kim <js1304@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: use system_highpri_wq for unbind_work

To speed cpu down processing up, use system_highpri_wq.
As scheduling priority of workers on it is higher than system_wq and
it is not contended by other normal works on this cpu, work on it
is processed faster than system_wq.

tj: CPU up/downs care quite a bit about latency these days.  This
    shouldn't hurt anything and makes sense.

Signed-off-by: Joonsoo Kim <js1304@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: fix checkpatch issues

Fixed some checkpatch warnings.

tj: adapted to wq/for-3.7 and massaged pr_xxx() format strings a bit.

Signed-off-by: Valentin Ilie <valentin.ilie@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
LKML-Reference: <1345326762-21747-1-git-send-email-valentin.ilie@gmail.com>

workqueue: make all workqueues non-reentrant

By default, each per-cpu part of a bound workqueue operates separately
and a work item may be executing concurrently on different CPUs.  The
behavior avoids some cross-cpu traffic but leads to subtle weirdities
and not-so-subtle contortions in the API.

* There's no sane usefulness in allowing a single work item to be
  executed concurrently on multiple CPUs.  People just get the
  behavior unintentionally and get surprised after learning about it.
  Most either explicitly synchronize or use non-reentrant/ordered
  workqueue but this is error-prone.

* flush_work() can't wait for multiple instances of the same work item
  on different CPUs.  If a work item is executing on cpu0 and then
  queued on cpu1, flush_work() can only wait for the one on cpu1.

  Unfortunately, work items can easily cross CPU boundaries
  unintentionally when the queueing thread gets migrated.  This means
  that if multiple queuers compete, flush_work() can't even guarantee
  that the instance queued right before it is finished before
  returning.

* flush_work_sync() was added to work around some of the deficiencies
  of flush_work().  In addition to the usual flushing, it ensures that
  all currently executing instances are finished before returning.
  This operation is expensive as it has to walk all CPUs and at the
  same time fails to address competing queuer case.

  Incorrectly using flush_work() when flush_work_sync() is necessary
  is an easy error to make and can lead to bugs which are difficult to
  reproduce.

* Similar problems exist for flush_delayed_work[_sync]().

Other than the cross-cpu access concern, there's no benefit in
allowing parallel execution and it's plain silly to have this level of
contortion for workqueue which is widely used from core code to
extremely obscure drivers.

This patch makes all workqueues non-reentrant.  If a work item is
executing on a different CPU when queueing is requested, it is always
queued to that CPU.  This guarantees that any given work item can be
executing on one CPU at maximum and if a work item is queued and
executing, both are on the same CPU.

The only behavior change which may affect workqueue users negatively
is that non-reentrancy overrides the affinity specified by
queue_work_on().  On a reentrant workqueue, the affinity specified by
queue_work_on() is always followed.  Now, if the work item is
executing on one of the CPUs, the work item will be queued there
regardless of the requested affinity.  I've reviewed all workqueue
users which request explicit affinity, and, fortunately, none seems to
be crazy enough to exploit parallel execution of the same work item.

This adds an additional busy_hash lookup if the work item was
previously queued on a different CPU.  This shouldn't be noticeable
under any sane workload.  Work item queueing isn't a very
high-frequency operation and they don't jump across CPUs all the time.
In a micro benchmark to exaggerate this difference - measuring the
time it takes for two work items to repeatedly jump between two CPUs a
number (10M) of times with busy_hash table densely populated, the
difference was around 3%.

While the overhead is measureable, it is only visible in pathological
cases and the difference isn't huge.  This change brings much needed
sanity to workqueue and makes its behavior consistent with timer.  I
think this is the right tradeoff to make.

This enables significant simplification of workqueue API.
Simplification patches will follow.

Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: gut flush[_delayed]_work_sync()

Now that all workqueues are non-reentrant, flush[_delayed]_work_sync()
are equivalent to flush[_delayed]_work().  Drop the separate
implementation and make them thin wrappers around
flush[_delayed]_work().

* start_flush_work() no longer takes @wait_executing as the only left
  user - flush_work() - always sets it to %true.

* __cancel_work_timer() uses flush_work() instead of wait_on_work().

Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: gut system_nrt[_freezable]_wq()

Now that all workqueues are non-reentrant, system[_freezable]_wq() are
equivalent to system_nrt[_freezable]_wq().  Replace the latter with
wrappers around system[_freezable]_wq().  The wrapping goes through
inline functions so that __deprecated can be added easily.

Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: cosmetic whitespace updates for macro definitions

Consistently use the last tab position for '\' line continuation in
complex macro definitions.  This is to help the following patches.

This patch is cosmetic.

Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: use hotcpu_notifier() for workqueue_cpu_down_callback()

workqueue_cpu_down_callback() is used only if HOTPLUG_CPU=y, so
hotcpu_notifier() fits better than cpu_notifier().

When HOTPLUG_CPU=y, hotcpu_notifier() and cpu_notifier() are the same.

When HOTPLUG_CPU=n, if we use cpu_notifier(),
workqueue_cpu_down_callback() will be called during boot to do
nothing, and the memory of workqueue_cpu_down_callback() and
gcwq_unbind_fn() will be discarded after boot.

If we use hotcpu_notifier(), we can avoid the no-op call of
workqueue_cpu_down_callback() and the memory of
workqueue_cpu_down_callback() and gcwq_unbind_fn() will be discard at
build time:

$ ls -l kernel/workqueue.o.cpu_notifier kernel/workqueue.o.hotcpu_notifier
-rw-rw-r-- 1 laijs laijs 484080 Sep 15 11:31 kernel/workqueue.o.cpu_notifier
-rw-rw-r-- 1 laijs laijs 478240 Sep 15 11:31 kernel/workqueue.o.hotcpu_notifier

$ size kernel/workqueue.o.cpu_notifier kernel/workqueue.o.hotcpu_notifier
   text	   data	    bss	    dec	    hex	filename
  18513	   2387	   1221	  22121	   5669	kernel/workqueue.o.cpu_notifier
  18082	   2355	   1221	  21658	   549a	kernel/workqueue.o.hotcpu_notifier

tj: Updated description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: reimplement cancel_delayed_work() using try_to_grab_pending()

cancel_delayed_work() can't be called from IRQ handlers due to its use
of del_timer_sync() and can't cancel work items which are already
transferred from timer to worklist.

Also, unlike other flush and cancel functions, a canceled delayed_work
would still point to the last associated cpu_workqueue.  If the
workqueue is destroyed afterwards and the work item is re-used on a
different workqueue, the queueing code can oops trying to dereference
already freed cpu_workqueue.

This patch reimplements cancel_delayed_work() using
try_to_grab_pending() and set_work_cpu_and_clear_pending().  This
allows the function to be called from IRQ handlers and makes its
behavior consistent with other flush / cancel functions.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>

workqueue: UNBOUND -> REBIND morphing in rebind_workers() should be atomic

The compiler may compile the following code into TWO write/modify
instructions.

	worker->flags &= ~WORKER_UNBOUND;
	worker->flags |= WORKER_REBIND;

so the other CPU may temporarily see worker->flags which doesn't have
either WORKER_UNBOUND or WORKER_REBIND set and perform local wakeup
prematurely.

Fix it by using single explicit assignment via ACCESS_ONCE().

Because idle workers have another WORKER_NOT_RUNNING flag, this bug
doesn't exist for them; however, update it to use the same pattern for
consistency.

tj: Applied the change to idle workers too and updated comments and
    patch description a bit.

Change-Id: I9b95f51d146c40c31ba028668d6f412bd74c6026
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org

workqueue: move WORKER_REBIND clearing in rebind_workers() to the end of the function

This doesn't make any functional difference and is purely to help the
next patch to be simpler.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>

workqueue: fix possible deadlock in idle worker rebinding

Currently, rebind_workers() and idle_worker_rebind() are two-way
interlocked.  rebind_workers() waits for idle workers to finish
rebinding and rebound idle workers wait for rebind_workers() to finish
rebinding busy workers before proceeding.

Unfortunately, this isn't enough.  The second wait from idle workers
is implemented as follows.

	wait_event(gcwq->rebind_hold, !(worker->flags & WORKER_REBIND));

rebind_workers() clears WORKER_REBIND, wakes up the idle workers and
then returns.  If CPU hotplug cycle happens again before one of the
idle workers finishes the above wait_event(), rebind_workers() will
repeat the first part of the handshake - set WORKER_REBIND again and
wait for the idle worker to finish rebinding - and this leads to
deadlock because the idle worker would be waiting for WORKER_REBIND to
clear.

This is fixed by adding another interlocking step at the end -
rebind_workers() now waits for all the idle workers to finish the
above WORKER_REBIND wait before returning.  This ensures that all
rebinding steps are complete on all idle workers before the next
hotplug cycle can happen.

This problem was diagnosed by Lai Jiangshan who also posted a patch to
fix the issue, upon which this patch is based.

This is the minimal fix and further patches are scheduled for the next
merge window to simplify the CPU hotplug path.

Signed-off-by: Tejun Heo <tj@kernel.org>
Original-patch-by: Lai Jiangshan <laijs@cn.fujitsu.com>
LKML-Reference: <1346516916-1991-3-git-send-email-laijs@cn.fujitsu.com>

workqueue: restore POOL_MANAGING_WORKERS

This patch restores POOL_MANAGING_WORKERS which was replaced by
pool->manager_mutex by 6037315269 "workqueue: use mutex for global_cwq
manager exclusion".

There's a subtle idle worker depletion bug across CPU hotplug events
and we need to distinguish an actual manager and CPU hotplug
preventing management.  POOL_MANAGING_WORKERS will be used for the
former and manager_mutex the later.

This patch just lays POOL_MANAGING_WORKERS on top of the existing
manager_mutex and doesn't introduce any synchronization changes.  The
next patch will update it.

Note that this patch fixes a non-critical anomaly where
too_many_workers() may return %true spuriously while CPU hotplug is in
progress.  While the issue could schedule idle timer spuriously, it
didn't trigger any actual misbehavior.

tj: Rewrote patch description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: fix possible idle worker depletion across CPU hotplug

To simplify both normal and CPU hotplug paths, worker management is
prevented while CPU hoplug is in progress.  This is achieved by CPU
hotplug holding the same exclusion mechanism used by workers to ensure
there's only one manager per pool.

If someone else seems to be performing the manager role, workers
proceed to execute work items.  CPU hotplug using the same mechanism
can lead to idle worker depletion because all workers could proceed to
execute work items while CPU hotplug is in progress and CPU hotplug
itself wouldn't actually perform the worker management duty - it
doesn't guarantee that there's an idle worker left when it releases
management.

This idle worker depletion, under extreme circumstances, can break
forward-progress guarantee and thus lead to deadlock.

This patch fixes the bug by using separate mechanisms for manager
exclusion among workers and hotplug exclusion.  For manager exclusion,
POOL_MANAGING_WORKERS which was restored by the previous patch is
used.  pool->manager_mutex is now only used for exclusion between the
elected manager and CPU hotplug.  The elected manager won't proceed
without holding pool->manager_mutex.

This ensures that the worker which won the manager position can't skip
managing while CPU hotplug is in progress.  It will block on
manager_mutex and perform management after CPU hotplug is complete.

Note that hotplug may happen while waiting for manager_mutex.  A
manager isn't either on idle or busy list and thus the hoplug code
can't unbind/rebind it.  Make the manager handle its own un/rebinding.

tj: Updated comment and description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: always clear WORKER_REBIND in busy_worker_rebind_fn()

busy_worker_rebind_fn() didn't clear WORKER_REBIND if rebinding failed
(CPU is down again).  This used to be okay because the flag wasn't
used for anything else.

However, after 25511a477 "workqueue: reimplement CPU online rebinding
to handle idle workers", WORKER_REBIND is also used to command idle
workers to rebind.  If not cleared, the worker may confuse the next
CPU_UP cycle by having REBIND spuriously set or oops / get stuck by
prematurely calling idle_worker_rebind().

  WARNING: at /work/os/wq/kernel/workqueue.c:1323 worker_thread+0x4cd/0x5
 00()
  Hardware name: Bochs
  Modules linked in: test_wq(O-)
  Pid: 33, comm: kworker/1:1 Tainted: G           O 3.6.0-rc1-work+ #3
  Call Trace:
   [<ffffffff8109039f>] warn_slowpath_common+0x7f/0xc0
   [<ffffffff810903fa>] warn_slowpath_null+0x1a/0x20
   [<ffffffff810b3f1d>] worker_thread+0x4cd/0x500
   [<ffffffff810bc16e>] kthread+0xbe/0xd0
   [<ffffffff81bd2664>] kernel_thread_helper+0x4/0x10
  ---[ end trace e977cf20f4661968 ]---
  BUG: unable to handle kernel NULL pointer dereference at           (null)
  IP: [<ffffffff810b3db0>] worker_thread+0x360/0x500
  PGD 0
  Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
  Modules linked in: test_wq(O-)
  CPU 0
  Pid: 33, comm: kworker/1:1 Tainted: G        W  O 3.6.0-rc1-work+ #3 Bochs Bochs
  RIP: 0010:[<ffffffff810b3db0>]  [<ffffffff810b3db0>] worker_thread+0x360/0x500
  RSP: 0018:ffff88001e1c9de0  EFLAGS: 00010086
  RAX: 0000000000000000 RBX: ffff88001e633e00 RCX: 0000000000004140
  RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000009
  RBP: ffff88001e1c9ea0 R08: 0000000000000000 R09: 0000000000000001
  R10: 0000000000000002 R11: 0000000000000000 R12: ffff88001fc8d580
  R13: ffff88001fc8d590 R14: ffff88001e633e20 R15: ffff88001e1c6900
  FS:  0000000000000000(0000) GS:ffff88001fc00000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
  CR2: 0000000000000000 CR3: 00000000130e8000 CR4: 00000000000006f0
  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
  Process kworker/1:1 (pid: 33, threadinfo ffff88001e1c8000, task ffff88001e1c6900)
  Stack:
   ffff880000000000 ffff88001e1c9e40 0000000000000001 ffff88001e1c8010
   ffff88001e519c78 ffff88001e1c9e58 ffff88001e1c6900 ffff88001e1c6900
   ffff88001e1c6900 ffff88001e1c6900 ffff88001fc8d340 ffff88001fc8d340
  Call Trace:
   [<ffffffff810bc16e>] kthread+0xbe/0xd0
   [<ffffffff81bd2664>] kernel_thread_helper+0x4/0x10
  Code: b1 00 f6 43 48 02 0f 85 91 01 00 00 48 8b 43 38 48 89 df 48 8b 00 48 89 45 90 e8 ac f0 ff ff 3c 01 0f 85 60 01 00 00 48 8b 53 50 <8b> 02 83 e8 01 85 c0 89 02 0f 84 3b 01 00 00 48 8b 43 38 48 8b
  RIP  [<ffffffff810b3db0>] worker_thread+0x360/0x500
   RSP <ffff88001e1c9de0>
  CR2: 0000000000000000

There was no reason to keep WORKER_REBIND on failure in the first
place - WORKER_UNBOUND is guaranteed to be set in such cases
preventing incorrectly activating concurrency management.  Always
clear WORKER_REBIND.

tj: Updated comment and description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: reimplement idle worker rebinding

Currently rebind_workers() uses rebinds idle workers synchronously
before proceeding to requesting busy workers to rebind.  This is
necessary because all workers on @worker_pool->idle_list must be bound
before concurrency management local wake-ups from the busy workers
take place.

Unfortunately, the synchronous idle rebinding is quite complicated.
This patch reimplements idle rebinding to simplify the code path.

Rather than trying to make all idle workers bound before rebinding
busy workers, we simply remove all to-be-bound idle workers from the
idle list and let them add themselves back after completing rebinding
(successful or not).

As only workers which finished rebinding can on on the idle worker
list, the idle worker list is guaranteed to have only bound workers
unless CPU went down again and local wake-ups are safe.

After the change, @worker_pool->nr_idle may deviate than the actual
number of idle workers on @worker_pool->idle_list.  More specifically,
nr_idle may be non-zero while ->idle_list is empty.  All users of
->nr_idle and ->idle_list are audited.  The only affected one is
too_many_workers() which is updated to check %false if ->idle_list is
empty regardless of ->nr_idle.

After this patch, rebind_workers() no longer performs the nasty
idle-rebind retries which require temporary release of gcwq->lock, and
both unbinding and rebinding are atomic w.r.t. global_cwq->lock.

worker->idle_rebind and global_cwq->rebind_hold are now unnecessary
and removed along with the definition of struct idle_rebind.

Changed from V1:
	1) remove unlikely from too_many_workers(), ->idle_list can be empty
	   anytime, even before this patch, no reason to use unlikely.
	2) fix a small rebasing mistake.
	   (which is from rebasing the orignal fixing patch to for-next)
	3) add a lot of comments.
	4) clear WORKER_REBIND unconditionaly in idle_worker_rebind()

tj: Updated comments and description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: WORKER_REBIND is no longer necessary for busy rebinding

Because the old unbind/rebinding implementation wasn't atomic w.r.t.
GCWQ_DISASSOCIATED manipulation which is protected by
global_cwq->lock, we had to use two flags, WORKER_UNBOUND and
WORKER_REBIND, to avoid incorrectly losing all NOT_RUNNING bits with
back-to-back CPU hotplug operations; otherwise, completion of
rebinding while another unbinding is in progress could clear UNBIND
prematurely.

Now that both unbind/rebinding are atomic w.r.t. GCWQ_DISASSOCIATED,
there's no need to use two flags.  Just one is enough.  Don't use
WORKER_REBIND for busy rebinding.

tj: Updated description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: WORKER_REBIND is no longer necessary for idle rebinding

Now both worker destruction and idle rebinding remove the worker from
idle list while it's still idle, so list_empty(&worker->entry) can be
used to test whether either is pending and WORKER_DIE to distinguish
between the two instead making WORKER_REBIND unnecessary.

Use list_empty(&worker->entry) to determine whether destruction or
rebinding is pending.  This simplifies worker state transitions.

WORKER_REBIND is not needed anymore.  Remove it.

tj: Updated comments and description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: rename manager_mutex to assoc_mutex

Now that manager_mutex's role has changed from synchronizing manager
role to excluding hotplug against manager, the name is misleading.

As it is protecting the CPU-association of the gcwq now, rename it to
assoc_mutex.

This patch is pure rename and doesn't introduce any functional change.

tj: Updated comments and description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: use __cpuinit instead of __devinit for cpu callbacks

For workqueue hotplug callbacks, it makes less sense to use __devinit
which discards the memory after boot if !HOTPLUG.  __cpuinit, which
discards the memory after boot if !HOTPLUG_CPU fits better.

tj: Updated description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: fix possible stall on try_to_grab_pending() of a delayed work item

Currently, when try_to_grab_pending() grabs a delayed work item, it
leaves its linked work items alone on the delayed_works.  The linked
work items are always NO_COLOR and will cause future
cwq_activate_first_delayed() increase cwq->nr_active incorrectly, and
may cause the whole cwq to stall.  For example,

state: cwq->max_active = 1, cwq->nr_active = 1
       one work in cwq->pool, many in cwq->delayed_works.

step1: try_to_grab_pending() removes a work item from delayed_works
       but leaves its NO_COLOR linked work items on it.

step2: Later on, cwq_activate_first_delayed() activates the linked
       work item increasing ->nr_active.

step3: cwq->nr_active = 1, but all activated work items of the cwq are
       NO_COLOR.  When they finish, cwq->nr_active will not be
       decreased due to NO_COLOR, and no further work items will be
       activated from cwq->delayed_works. the cwq stalls.

Fix it by ensuring the target work item is activated before stealing
PENDING in try_to_grab_pending().  This ensures that all the linked
work items are activated without incorrectly bumping cwq->nr_active.

tj: Updated comment and description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@kernel.org

workqueue: reimplement work_on_cpu() using system_wq

The existing work_on_cpu() implementation is hugely inefficient.  It
creates a new kthread, execute that single function and then let the
kthread die on each invocation.

Now that system_wq can handle concurrent executions, there's no
advantage of doing this.  Reimplement work_on_cpu() using system_wq
which makes it simpler and way more efficient.

stable: While this isn't a fix in itself, it's needed to fix a
        workqueue related bug in cpufreq/powernow-k8.  AFAICS, this
        shouldn't break other existing users.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Len Brown <lenb@kernel.org>
Cc: Rafael J. Wysocki <rjw@sisk.pl>
Cc: stable@vger.kernel.org

workqueue: introduce cwq_set_max_active() helper for thaw_workqueues()

Using a helper instead of open code makes thaw_workqueues() clearer.
The helper will also be used by the next patch.

tj: Slight update to comment and description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: use cwq_set_max_active() helper for workqueue_set_max_active()

workqueue_set_max_active() may increase ->max_active without
activating delayed works and may make the activation order differ from
the queueing order.  Both aren't strictly bugs but the resulting
behavior could be a bit odd.

To make things more consistent, use cwq_set_max_active() helper which
immediately makes use of the newly increased max_mactive if there are
delayed work items and also keeps the activation order.

tj: Slight update to description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: remove spurious WARN_ON_ONCE(in_irq()) from try_to_grab_pending()

e0aecdd874 ("workqueue: use irqsafe timer for delayed_work") made
try_to_grab_pending() safe to use from irq context but forgot to
remove WARN_ON_ONCE(in_irq()).  Remove it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>

workqueue: cancel_delayed_work() should return %false if work item is idle

57b30ae77b ("workqueue: reimplement cancel_delayed_work() using
try_to_grab_pending()") made cancel_delayed_work() always return %true
unless someone else is also trying to cancel the work item, which is
broken - if the target work item is idle, the return value should be
%false.

try_to_grab_pending() indicates that the target work item was idle by
zero return value.  Use it for return.  Note that this brings
cancel_delayed_work() in line with __cancel_work_timer() in return
value handling.

Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
LKML-Reference: <444a6439-b1a4-4740-9e7e-bc37267cfe73@default>

workqueue: exit rescuer_thread() as TASK_RUNNING

A rescue thread exiting TASK_INTERRUPTIBLE can lead to a task scheduling
off, never to be seen again.  In the case where this occurred, an exiting
thread hit reiserfs homebrew conditional resched while holding a mutex,
bringing the box to its knees.

PID: 18105  TASK: ffff8807fd412180  CPU: 5   COMMAND: "kdmflush"
 #0 [ffff8808157e7670] schedule at ffffffff8143f489
 #1 [ffff8808157e77b8] reiserfs_get_block at ffffffffa038ab2d [reiserfs]
 #2 [ffff8808157e79a8] __block_write_begin at ffffffff8117fb14
 #3 [ffff8808157e7a98] reiserfs_write_begin at ffffffffa0388695 [reiserfs]
 #4 [ffff8808157e7ad8] generic_perform_write at ffffffff810ee9e2
 #5 [ffff8808157e7b58] generic_file_buffered_write at ffffffff810eeb41
 #6 [ffff8808157e7ba8] __generic_file_aio_write at ffffffff810f1a3a
 #7 [ffff8808157e7c58] generic_file_aio_write at ffffffff810f1c88
 #8 [ffff8808157e7cc8] do_sync_write at ffffffff8114f850
 #9 [ffff8808157e7dd8] do_acct_process at ffffffff810a268f
    [exception RIP: kernel_thread_helper]
    RIP: ffffffff8144a5c0  RSP: ffff8808157e7f58  RFLAGS: 00000202
    RAX: 0000000000000000  RBX: 0000000000000000  RCX: 0000000000000000
    RDX: 0000000000000000  RSI: ffffffff8107af60  RDI: ffff8803ee491d18
    RBP: 0000000000000000   R8: 0000000000000000   R9: 0000000000000000
    R10: 0000000000000000  R11: 0000000000000000  R12: 0000000000000000
    R13: 0000000000000000  R14: 0000000000000000  R15: 0000000000000000
    ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018

Signed-off-by: Mike Galbraith <mgalbraith@suse.de>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org

workqueue: mod_delayed_work_on() shouldn't queue timer on 0 delay

8376fe22c7 ("workqueue: implement mod_delayed_work[_on]()")
implemented mod_delayed_work[_on]() using the improved
try_to_grab_pending().  The function is later used, among others, to
replace [__]candel_delayed_work() + queue_delayed_work() combinations.

Unfortunately, a delayed_work item w/ zero @delay is handled slightly
differently by mod_delayed_work_on() compared to
queue_delayed_work_on().  The latter skips timer altogether and
directly queues it using queue_work_on() while the former schedules
timer which will expire on the closest tick.  This means, when @delay
is zero, that [__]cancel_delayed_work() + queue_delayed_work_on()
makes the target item immediately executable while
mod_delayed_work_on() may induce delay of upto a full tick.

This somewhat subtle difference breaks some of the converted users.
e.g. block queue plugging uses delayed_work for deferred processing
and uses mod_delayed_work_on() when the queue needs to be immediately
unplugged.  The above problem manifested as noticeably higher number
of context switches under certain circumstances.

The difference in behavior was caused by missing special case handling
for 0 delay in mod_delayed_work_on() compared to
queue_delayed_work_on().  Joonsoo Kim posted a patch to add it -
("workqueue: optimize mod_delayed_work_on() when @delay == 0")[1].
The patch was queued for 3.8 but it was described as optimization and
I missed that it was a correctness issue.

As both queue_delayed_work_on() and mod_delayed_work_on() use
__queue_delayed_work() for queueing, it seems that the better approach
is to move the 0 delay special handling to the function instead of
duplicating it in mod_delayed_work_on().

Fix the problem by moving 0 delay special case handling from
queue_delayed_work_on() to __queue_delayed_work().  This replaces
Joonsoo's patch.

[1] http://thread.gmane.org/gmane.linux.kernel/1379011/focus=1379012

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-and-tested-by: Anders Kaseorg <andersk@MIT.EDU>
Reported-and-tested-by: Zlatko Calusic <zlatko.calusic@iskon.hr>
LKML-Reference: <alpine.DEB.2.00.1211280953350.26602@dr-wily.mit.edu>
LKML-Reference: <50A78AA9.5040904@iskon.hr>
Cc: Joonsoo Kim <js1304@gmail.com>

workqueue: trivial fix for return statement in work_busy()

Return type of work_busy() is unsigned int.
There is return statement returning boolean value, 'false' in work_busy().
It is not problem, because 'false' may be treated '0'.
However, fixing it would make code robust.

Signed-off-by: Joonsoo Kim <js1304@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: add WARN_ON_ONCE() on CPU number to wq_worker_waking_up()

Recently, workqueue code has gone through some changes and we found
some bugs related to concurrency management operations happening on
the wrong CPU.  When a worker is concurrency managed
(!WORKER_NOT_RUNNIG), it should be bound to its associated cpu and
woken up to that cpu.  Add WARN_ON_ONCE() to verify this.

Signed-off-by: Joonsoo Kim <js1304@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

workqueue: convert BUG_ON()s in __queue_delayed_work() to WARN_ON_ONCE()s

8852aac25e ("workqueue: mod_delayed_work_on() shouldn't queue timer on
0 delay") unexpectedly uncovered a very nasty abuse of delayed_work in
megaraid - it allocated work_struct, casted it to delayed_work and
then pass that into queue_delayed_work().

Previously, this was okay because 0 @delay short-circuited to
queue_work() before doing anything with delayed_work.  8852aac25e
moved 0 @delay test into __queue_delayed_work() after sanity check on
delayed_work making megaraid trigger BUG_ON().

Although megaraid is already fixed by c1d390d8e6 ("megaraid: fix
BUG_ON() from incorrect use of delayed work"), this patch converts
BUG_ON()s in __queue_delayed_work() to WARN_ON_ONCE()s so that such
abusers, if there are more, trigger warning but don't crash the
machine.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Xiaotian Feng <xtfeng@gmail.com>

wq

Change-Id: Ia3c507777a995f32bf6b40dc8318203e53134229