[PATCH] workqueue: ensure @task is valid across kthread_stop()

[Lai Jiangshan]

When a worker is set WORKER_DIE, it may die very quickly,
and kthread_stop() will access to a stale task stuct/stack.
To avoid this, we use get_task_struct() to ensure @task is valid.

See more comments in kthread_create_on_node()&kthread_stop().
Note: comments in kthread_create_on_node() does not elaborate
any use case like this patch, but it is a valid way to use
kthread_stop().

CC: stable@xxxxxxxxxxxxxxx
Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
---
 kernel/workqueue.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 82ef9f3..783d5f2 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1856,9 +1856,12 @@ static void destroy_worker(struct worker *worker)
 
 	idr_remove(&pool->worker_idr, worker->id);
 
+	/* Enusre the @worker->task is valid across kthread_stop() */
+	get_task_struct(worker->task);
 	spin_unlock_irq(&pool->lock);
 
 	kthread_stop(worker->task);
+	put_task_struct(worker->task);
 	kfree(worker);
 
 	spin_lock_irq(&pool->lock);
-- 
1.7.7.6

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