linux-kernel - [PATCH 3.11 055/131] workqueue: ensure @task is valid across kthread

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Message-Id: <1394029455-23128-56-git-send-email-luis.henriques@canonical.com>
Date:	Wed,  5 Mar 2014 14:22:59 +0000
From:	Luis Henriques <luis.henriques@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Lai Jiangshan <laijs@...fujitsu.com>, Tejun Heo <tj@...nel.org>,
	Luis Henriques <luis.henriques@...onical.com>
Subject: [PATCH 3.11 055/131] workqueue: ensure @task is valid across kthread_stop()

3.11.10.6 -stable review patch.  If anyone has any objections, please let me know.

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

From: Lai Jiangshan <laijs@...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@...fujitsu.com>
Signed-off-by: Tejun Heo <tj@...nel.org>
Signed-off-by: Luis Henriques <luis.henriques@...onical.com>
---
 kernel/workqueue.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 4235d5e..d06e6e1 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1836,6 +1836,12 @@ 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;
 
@@ -1844,6 +1850,7 @@ 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);
-- 
1.9.0

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