mutex: Place lock in contended state after fastpath_lock failure

commit 0bce9c46bf3b15f485d82d7e81dabed6ebcc24b1 upstream.

ARM recently moved to asm-generic/mutex-xchg.h for its mutex
implementation after the previous implementation was found to be missing
some crucial memory barriers. However, this has revealed some problems
running hackbench on SMP platforms due to the way in which the
MUTEX_SPIN_ON_OWNER code operates.

The symptoms are that a bunch of hackbench tasks are left waiting on an
unlocked mutex and therefore never get woken up to claim it. This boils
down to the following sequence of events:

        Task A        Task B        Task C        Lock value
0                                                     1
1       lock()                                        0
2                     lock()                          0
3                     spin(A)                         0
4       unlock()                                      1
5                                   lock()            0
6                     cmpxchg(1,0)                    0
7                     contended()                    -1
8       lock()                                        0
9       spin(C)                                       0
10                                  unlock()          1
11      cmpxchg(1,0)                                  0
12      unlock()                                      1

At this point, the lock is unlocked, but Task B is in an uninterruptible
sleep with nobody to wake it up.

This patch fixes the problem by ensuring we put the lock into the
contended state if we fail to acquire it on the fastpath, ensuring that
any blocked waiters are woken up when the mutex is released.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Chris Mason <chris.mason@fusionio.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: <stable@vger.kernel.org>
Reviewed-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-6e9lrw2avczr0617fzl5vqb8@git.kernel.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
writeback: remove wb_list

The wb_list member of struct backing_device_info always has exactly one
element.  Just use the direct bdi->wb pointer instead and simplify some
code.

Also remove bdi_task_init which is now trivial to prepare for the next
patch.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
writeback: merge bdi_writeback_task and bdi_start_fn

Move all code for the writeback thread into fs/fs-writeback.c instead of
splitting it over two functions in two files.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
writeback: Initial tracing support

Trace queue/sched/exec parts of the writeback loop. This provides
insight into when and why flusher threads are scheduled to run. e.g
a sync invocation leaves traces like:

     sync-[...]: writeback_queue: bdi 8:0: sb_dev 8:1 nr_pages=7712 sync_mode=0 kupdate=0 range_cyclic=0 background=0
flush-8:0-[...]: writeback_exec: bdi 8:0: sb_dev 8:1 nr_pages=7712 sync_mode=0 kupdate=0 range_cyclic=0 background=0

This also lays the foundation for adding more writeback tracing to
provide deeper insight into the whole writeback path.

The original tracing code is from Jens Axboe, though this version is
a rewrite as a result of the code being traced changing
significantly.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
writeback: Add tracing to balance_dirty_pages

Tracing high level background writeback events is good, but it doesn't
give the entire picture. Add visibility into write throttling to catch IO
dispatched by foreground throttling of processing dirtying lots of pages.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
writeback: remove wb in get_next_work_item

83ba7b07 cleans up the writeback.
So we don't use wb any more in get_next_work_item.
Let's remove unnecessary argument.

CC: Christoph Hellwig <hch@lst.de>
Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
writeback: harmonize writeback threads naming

The write-back code mixes words "thread" and "task" for the same things. This
is not a big deal, but still an inconsistency.

hch: a convention I tend to use and I've seen in various places
is to always use _task for the storage of the task_struct pointer,
and thread everywhere else.  This especially helps with having
foo_thread for the actual thread and foo_task for a global
variable keeping the task_struct pointer

This patch renames:
* 'bdi_add_default_flusher_task()' -> 'bdi_add_default_flusher_thread()'
* 'bdi_forker_task()'              -> 'bdi_forker_thread()'

because bdi threads are 'bdi_writeback_thread()', so these names are more
consistent.

This patch also amends commentaries and makes them refer the forker and bdi
threads as "thread", not "task".

Also, while on it, make 'bdi_add_default_flusher_thread()' declaration use
'static void' instead of 'void static' and make checkpatch.pl happy.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
writeback: fix possible race when creating bdi threads

This patch fixes a very unlikely race condition on the bdi forker thread error
path: when bdi thread creation fails, 'bdi->wb.task' may contain the error code
for a short period of time. If at the same time someone submits a work to this
bdi, we can end up with an oops 'bdi_queue_work()' while executing
'wake_up_process(wb->task)'.

This patch fixes the issue by introducing a temporary variable 'task' and
storing the possible error code there, so that 'wb->task' would never take
erroneous values.

Note, this race is very unlikely and I never hit it, so it is theoretical, but
nevertheless worth fixing.

This patch also merges 2 comments which were previously separate.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
writeback: do not lose wake-ups in the forker thread - 1

Currently the forker thread can lose wake-ups which may lead to unnecessary
delays in processing bdi works. E.g., consider the following scenario.

1. 'bdi_forker_thread()' walks the 'bdi_list', finds out there is nothing to
   do, and is about to finish the loop.
2. A bdi thread decides to exit because it was inactive for long time.
3. 'bdi_queue_work()' adds a work to the bdi which just exited, so it wakes up
   the forker thread.
4. but 'bdi_forker_thread()' executes 'set_current_state(TASK_INTERRUPTIBLE)'
   and goes sleep. We lose a wake-up.

Losing the wake-up is not fatal, but this means that the bdi work processing
will be delayed by up to 5 sec. This race is theoretical, I never hit it, but
it is worth fixing.

The fix is to execute 'set_current_state(TASK_INTERRUPTIBLE)' _before_ walking
'bdi_list', not after.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
writeback: do not lose wake-ups in the forker thread - 2

Currently, if someone submits jobs for the default bdi, we can lose wake-up
events. E.g., this can happen if 'bdi_queue_work()' is called when
'bdi_forker_thread()' is executing code after 'wb_do_writeback(me, 0)', but
before 'set_current_state(TASK_INTERRUPTIBLE)'.

This situation is unlikely, and the result is not very severe - we'll just
delay the execution of the work, but this is still not very nice.

This patch fixes the issue by checking whether the default bdi has works before
the forker thread goes sleep.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
writeback: do not lose wake-ups in bdi threads

Currently, bdi threads ('bdi_writeback_thread()') can lose wake-ups. For
example, if 'bdi_queue_work()' is executed after the bdi thread have had
finished 'wb_do_writeback()' but before it called
'schedule_timeout_interruptible()'.

To fix this issue, we have to check whether we have works to process after we
have changed the task state to 'TASK_INTERRUPTIBLE'.

This patch also clean-ups handling of the cases when 'dirty_writeback_interval'
is zero or non-zero.

Additionally, this patch also removes unneeded 'list_empty_careful()' call.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
writeback: simplify bdi code a little

This patch simplifies bdi code a little by removing the 'pending_list' which is
redundant. Indeed, currently the forker thread ('bdi_forker_thread()') is
working like this:

1. In a loop, fetch all bdi's which have works but have no writeback thread and
   move them to the 'pending_list'.
2. If the list is empty, sleep for 5 sec.
3. Otherwise, take one bdi from the list, fork the writeback thread for this
   bdi, and repeat the loop.

IOW, it first moves everything to the 'pending_list', then process only one
element, and so on. This patch simplifies the algorithm, which is now as
follows.

1. Find the first bdi which has a work and remove it from the global list of
   bdi's (bdi_list).
2. If there was not such bdi, sleep 5 sec.
3. Fork the writeback thread for this bdi and repeat the loop.

IOW, now we find the first bdi to process, process it, and so on. This is
simpler and involves less lists.

The bonus now is that we can get rid of a couple of functions, as well as
remove complications which involve 'rcu_call()' and 'bdi->rcu_head'.

This patch also makes sure we use 'list_add_tail_rcu()', instead of plain
'list_add_tail()', but this piece of code is going to be removed in the next
patch anyway.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
writeback: do not remove bdi from bdi_list

The forker thread removes bdis from 'bdi_list' before forking the bdi thread.
But this is wrong for at least 2 reasons.

Reason #1: if we temporary remove a bdi from the list, we may miss works which
           would otherwise be given to us.

Reason #2: this is racy; indeed, 'bdi_wb_shutdown()' expects that bdis are
           always in the 'bdi_list' (see 'bdi_remove_from_list()'), and when
           it races with the forker thread, it can shut down the bdi thread
           at the same time as the forker creates it.

This patch makes sure the forker thread never removes bdis from 'bdi_list'
(which was suggested by Christoph Hellwig).

In order to make sure that we do not race with 'bdi_wb_shutdown()', we have to
hold the 'bdi_lock' while walking the 'bdi_list' and setting the 'BDI_pending'
flag.

NOTE! The error path is interesting. Currently, when we fail to create a bdi
thread, we move the bdi to the tail of 'bdi_list'. But if we never remove the
bdi from the list, we cannot move it to the tail either, because then we can
mess up the RCU readers which walk the list. And also, we'll have the race
described above in "Reason #2".

But I not think that adding to the tail is any important so I just do not do
that.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
writeback: move last_active to bdi

Currently bdi threads use local variable 'last_active' which stores last time
when the bdi thread did some useful work. Move this local variable to 'struct
bdi_writeback'. This is just a preparation for the further patches which will
make the forker thread decide when bdi threads should be killed.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
writeback: restructure bdi forker loop a little

This patch re-structures the bdi forker a little:
1. Add 'bdi_cap_flush_forker(bdi)' condition check to the bdi loop. The reason
   for this is that the forker thread can start _before_ the 'BDI_registered'
   flag is set (see 'bdi_register()'), so the WARN() statement will fire for
   the default bdi. I observed this warning at boot-up.

2. Introduce an enum 'action' and use "switch" statement in the outer loop.
   This is a preparation to the further patch which will teach the forker
   thread killing bdi threads, so we'll have another case in the "switch"
   statement. This change was suggested by Christoph Hellwig.

This patch is just a small step towards the coming change where the forker
thread will kill the bdi threads. It should simplify reviewing the following
changes, which would otherwise be larger.

This patch also amends comments a little.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
writeback: move bdi threads exiting logic to the forker thread

Currently, bdi threads can decide to exit if there were no useful activities
for 5 minutes. However, this causes nasty races: we can easily oops in the
'bdi_queue_work()' if the bdi thread decides to exit while we are waking it up.

And even if we do not oops, but the bdi tread exits immediately after we wake
it up, we'd lose the wake-up event and have an unnecessary delay (up to 5 secs)
in the bdi work processing.

This patch makes the forker thread to be the central place which not only
creates bdi threads, but also kills them if they were inactive long enough.
This better design-wise.

Another reason why this change was done is to prepare for the further changes
which will prevent the bdi threads from waking up every 5 sec and wasting
power. Indeed, when the task does not wake up periodically anymore, it won't be
able to exit either.

This patch also moves the the 'wake_up_bit()' call from the bdi thread to the
forker thread as well. So now the forker thread sets the BDI_pending bit, then
forks the task or kills it, then clears the bit and wakes up the waiting
process.

The only process which may wain on the bit is 'bdi_wb_shutdown()'. This
function was changed as well - now it first removes the bdi from the
'bdi_list', then waits on the 'BDI_pending' bit. Once it wakes up, it is
guaranteed that the forker thread won't race with it, because the bdi is not
visible. Note, the forker thread sets the 'BDI_pending' bit under the
'bdi->wb_lock' which is essential for proper serialization.

And additionally, when we change 'bdi->wb.task', we now take the
'bdi->work_lock', to make sure that we do not lose wake-ups which we otherwise
would when raced with, say, 'bdi_queue_work()'.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
writeback: prevent unnecessary bdi threads wakeups

Finally, we can get rid of unnecessary wake-ups in bdi threads, which are very
bad for battery-driven devices.

There are two types of activities bdi threads do:
1. process bdi works from the 'bdi->work_list'
2. periodic write-back

So there are 2 sources of wake-up events for bdi threads:

1. 'bdi_queue_work()' - submits bdi works
2. '__mark_inode_dirty()' - adds dirty I/O to bdi's

The former already has bdi wake-up code. The latter does not, and this patch
adds it.

'__mark_inode_dirty()' is hot-path function, but this patch adds another
'spin_lock(&bdi->wb_lock)' there. However, it is taken only in rare cases when
the bdi has no dirty inodes. So adding this spinlock should be fine and should
not affect performance.

This patch makes sure bdi threads and the forker thread do not wake-up if there
is nothing to do. The forker thread will nevertheless wake up at least every
5 min. to check whether it has to kill a bdi thread. This can also be optimized,
but is not worth it.

This patch also tidies up the warning about unregistered bid, and turns it from
an ugly crocodile to a simple 'WARN()' statement.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
writeback: optimize periodic bdi thread wakeups

Whe the first inode for a bdi is marked dirty, we wake up the bdi thread which
should take care of the periodic background write-out. However, the write-out
will actually start only 'dirty_writeback_interval' centisecs later, so we can
delay the wake-up.

This change was requested by Nick Piggin who pointed out that if we delay the
wake-up, we weed out 2 unnecessary contex switches, which matters because
'__mark_inode_dirty()' is a hot-path function.

This patch introduces a new function - 'bdi_wakeup_thread_delayed()', which
sets up a timer to wake-up the bdi thread and returns. So the wake-up is
delayed.

We also delete the timer in bdi threads just before writing-back. And
synchronously delete it when unregistering bdi. At the unregister point the bdi
does not have any users, so no one can arm it again.

Since now we take 'bdi->wb_lock' in the timer, which can execute in softirq
context, we have to use 'spin_lock_bh()' for 'bdi->wb_lock'. This patch makes
this change as well.

This patch also moves the 'bdi_wb_init()' function down in the file to avoid
forward-declaration of 'bdi_wakeup_thread_delayed()'.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
writeback: remove unnecessary init_timer call

The 'setup_timer()' function also calls 'init_timer()', so the extra
'init_timer()' call is not needed. Indeed, 'setup_timer()' is basically
'init_timer()' plus callback function and data pointers initialization.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
writeback: add new tracepoints

Add 2 new trace points to the periodic write-back wake up case, just like we do
in the 'bdi_queue_work()' function. Namely, introduce:

1. trace_writeback_wake_thread(bdi)
2. trace_writeback_wake_forker_thread(bdi)

The first event is triggered every time we wake up a bdi thread to start
periodic background write-out. The second event is triggered only when the bdi
thread does not exist and should be created by the forker thread.

This patch was suggested by Dave Chinner and Christoph Hellwig.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>