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>

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 82ef9f3..193e977 100644 (file)
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1851,6 +1851,12 @@ 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;
 
@@ -1859,6 +1865,7 @@ static void destroy_worker(struct worker *worker)
        spin_unlock_irq(&pool->lock);
 
        kthread_stop(worker->task);
+       put_task_struct(worker->task);
        kfree(worker);
 
        spin_lock_irq(&pool->lock);