linux-kernel - [PATCH v2 1/1] workqueue: make the pwq->stats[] increment atomic

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Date:   Fri, 18 Aug 2023 21:48:59 +0200
From:   Mirsad Goran Todorovac <mirsad.todorovac@....unizg.hr>
To:     Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Cc:     Lai Jiangshan <jiangshanlai@...il.com>,
        Mirsad Goran Todorovac <mirsad.todorovac@....unizg.hr>
Subject: [PATCH v2 1/1] workqueue: make the pwq->stats[] increment atomic

KCSAN has discovered a data race in kernel/workqueue.c:2598:

[ 1863.554079] ==================================================================
[ 1863.554118] BUG: KCSAN: data-race in process_one_work / process_one_work

[ 1863.554142] write to 0xffff963d99d79998 of 8 bytes by task 5394 on cpu 27:
[ 1863.554154] process_one_work (kernel/workqueue.c:2598)
[ 1863.554166] worker_thread (./include/linux/list.h:292 kernel/workqueue.c:2752)
[ 1863.554177] kthread (kernel/kthread.c:389)
[ 1863.554186] ret_from_fork (arch/x86/kernel/process.c:145)
[ 1863.554197] ret_from_fork_asm (arch/x86/entry/entry_64.S:312)

[ 1863.554213] read to 0xffff963d99d79998 of 8 bytes by task 5450 on cpu 12:
[ 1863.554224] process_one_work (kernel/workqueue.c:2598)
[ 1863.554235] worker_thread (./include/linux/list.h:292 kernel/workqueue.c:2752)
[ 1863.554247] kthread (kernel/kthread.c:389)
[ 1863.554255] ret_from_fork (arch/x86/kernel/process.c:145)
[ 1863.554266] ret_from_fork_asm (arch/x86/entry/entry_64.S:312)

[ 1863.554280] value changed: 0x0000000000001766 -> 0x000000000000176a

[ 1863.554295] Reported by Kernel Concurrency Sanitizer on:
[ 1863.554303] CPU: 12 PID: 5450 Comm: kworker/u64:1 Tainted: G             L     6.5.0-rc6+ #44
[ 1863.554314] Hardware name: ASRock X670E PG Lightning/X670E PG Lightning, BIOS 1.21 04/26/2023
[ 1863.554322] Workqueue: btrfs-endio btrfs_end_bio_work [btrfs]
[ 1863.554941] ==================================================================

    lockdep_invariant_state(true);
→   pwq->stats[PWQ_STAT_STARTED]++;
    trace_workqueue_execute_start(work);
    worker->current_func(work);

The quick and dirty solution is to use atomic_inc():

    lockdep_invariant_state(true);
→   atomic_inc((atomic_t *) &pwq->stats[PWQ_STAT_STARTED]);
    trace_workqueue_execute_start(work);
    worker->current_func(work);

There seem to be the other instances of the similar use of pwq->stats, but they need separate
patch and testing.

Fixes: 725e8ec59c56c ("workqueue: Add pwq->stats[] and a monitoring script")
Cc: Tejun Heo <tj@...nel.org>
Cc: Lai Jiangshan <jiangshanlai@...il.com>
Signed-off-by: Mirsad Goran Todorovac <mirsad.todorovac@....unizg.hr>
---
 v2:
	fixed typo in the Subject line, commit unchanged.

 v1:
	quick and dirty fix (RFC).

 kernel/workqueue.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 800b4208dba9..237e5800fbad 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2595,7 +2595,7 @@ __acquires(&pool->lock)
 	 * workqueues), so hiding them isn't a problem.
 	 */
 	lockdep_invariant_state(true);
-	pwq->stats[PWQ_STAT_STARTED]++;
+	atomic_inc((atomic_t *) &pwq->stats[PWQ_STAT_STARTED]);
 	trace_workqueue_execute_start(work);
 	worker->current_func(work);
 	/*
-- 
2.34.1