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workqueue: add missing wmb() in clear_work_data()

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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>
This commit is contained in:
Tejun Heo 2012-08-13 17:08:19 -07:00
parent 1265057fa0
commit 23657bb192
1 changed files with 12 additions and 7 deletions
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19
kernel/workqueue.c
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@ -570,11 +570,19 @@ static void set_work_cwq(struct work_struct *work,
static void set_work_cpu_and_clear_pending(struct work_struct *work,
unsigned int cpu)
{
/*
* The following wmb is paired with the implied mb in
* test_and_set_bit(PENDING) and ensures all updates to @work made
* here are visible to and precede any updates by the next PENDING
* owner.
*/
smp_wmb();
set_work_data(work, (unsigned long)cpu << WORK_OFFQ_CPU_SHIFT, 0);
}
static void clear_work_data(struct work_struct *work)
{
smp_wmb(); /* see set_work_cpu_and_clear_pending() */
set_work_data(work, WORK_STRUCT_NO_CPU, 0);
}
@ -2182,14 +2190,11 @@ __acquires(&gcwq->lock)
wake_up_worker(pool);
/*
* Record the last CPU and clear PENDING. The following wmb is
* paired with the implied mb in test_and_set_bit(PENDING) and
* ensures all updates to @work made here are visible to and
* precede any updates by the next PENDING owner. Also, clear
* PENDING inside @gcwq->lock so that PENDING and queued state
* changes happen together while IRQ is disabled.
* Record the last CPU and clear PENDING which should be the last
* update to @work. Also, do this inside @gcwq->lock so that
* PENDING and queued state changes happen together while IRQ is
* disabled.
*/
smp_wmb();
set_work_cpu_and_clear_pending(work, gcwq->cpu);
spin_unlock_irq(&gcwq->lock);