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Subject[PATCH 3.8 39/81] workqueue: ensure @task is valid across kthread_stop()
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3.8.13.20 -stable review patch.  If anyone has any objections, please let me know.

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

From: Lai Jiangshan <laijs@cn.fujitsu.com>

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@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
---
kernel/workqueue.c | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 338b3bd..1fd7e24 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1902,12 +1902,19 @@ static void destroy_worker(struct worker *worker)
if (worker->flags & WORKER_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);
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);
--
1.8.3.2


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