Commit 5bdfff96 authored Feb 15, 2014 by Lai Jiangshan Committed by Tejun Heo Feb 18, 2014

workqueue: ensure @task is valid across kthread_stop()



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.

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

parent fada94ee

kernel/workqueue.c

+7 −0

Original line number	Original line	Diff line number	Diff line
	@@ -1851,6 +1851,12 @@ static void destroy_worker(struct worker *worker)
	if (worker->flags & WORKER_IDLE)		if (worker->flags & WORKER_IDLE)
	pool->nr_idle--;		pool->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);		list_del_init(&worker->entry);
	worker->flags \|= WORKER_DIE;		worker->flags \|= WORKER_DIE;

	@@ -1859,6 +1865,7 @@ static void destroy_worker(struct worker *worker)
	spin_unlock_irq(&pool->lock);		spin_unlock_irq(&pool->lock);

	kthread_stop(worker->task);		kthread_stop(worker->task);
			put_task_struct(worker->task);
	kfree(worker);		kfree(worker);

	spin_lock_irq(&pool->lock);		spin_lock_irq(&pool->lock);

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