[PATCH 3.2 128/200] workqueue: ensure @task is valid across kthread_stop()

[Ben Hutchings]

3.2.56-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>

commit 5bdfff96c69a4d5ab9c49e60abf9e070ecd2acbb 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@xxxxxxxxxxxxxx>
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 kernel/workqueue.c | 7 +++++++
 1 file changed, 7 insertions(+)

--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1474,12 +1474,19 @@ static void destroy_worker(struct 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);

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