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workqueue: ensure @task is valid across kthread_stop()

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commit 5bdfff96c6 upstream.

When a kworker should die, the kworkre is notified through WORKER_DIE
flag instead of kthread_should_stop().  This, IIRC, is primarily to
keep the test synchronized inside worker_pool lock.  WORKER_DIE is
first set while holding pool->lock, the lock is dropped and
kthread_stop() is called.

Unfortunately, this means that there's a slight chance that the target
kworker may see WORKER_DIE before kthread_stop() finishes and exits
and frees the target task before or during kthread_stop().

Fix it by pinning the target task before setting WORKER_DIE and
putting it after kthread_stop() is done.

tj: Improved patch description and comment.  Moved pinning above
    WORKER_DIE for better signify what it's protecting.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Lai Jiangshan 2014-02-15 22:02:28 +08:00 committed by Greg Kroah-Hartman
parent 8d36299431
commit 23f0913c2b
1 changed files with 7 additions and 0 deletions
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7
kernel/workqueue.c
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@ -1469,12 +1469,19 @@ static void destroy_worker(struct worker *worker)
if (worker->flags & WORKER_IDLE)
gcwq->nr_idle--;
/*
* Once WORKER_DIE is set, the kworker may destroy itself at any
* point. Pin to ensure the task stays until we're done with it.
*/
get_task_struct(worker->task);
list_del_init(&worker->entry);
worker->flags |= WORKER_DIE;
spin_unlock_irq(&gcwq->lock);
kthread_stop(worker->task);
put_task_struct(worker->task);
kfree(worker);
spin_lock_irq(&gcwq->lock);