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>
Signed-off-by: Blechd0se <alex.christ@hotmail.de>
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>
Signed-off-by: Blechd0se <alex.christ@hotmail.de>
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>
Signed-off-by: Blechd0se <alex.christ@hotmail.de>
workqueue: make morphing in rebind_workers() atomic

Commit 6adebb0e1d4820435e0c6739b0de20a196cc20d5 didn't quite change the main purpose.
This is actually some sort of port over from the upstream kernel;
https://github.com/torvalds/linux/commit/96e65306b81351b656835c15931d1d237b252f27

Signed-off-by: Blechd0se <alex.christ@hotmail.de>
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>
Signed-off-by: Blechd0se <alex.christ@hotmail.de>
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>
Signed-off-by: Blechd0se <alex.christ@hotmail.de>
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>
Signed-off-by: Blechd0se <alex.christ@hotmail.de>