linux-kernel - [tip: locking/core] locking: Mark racy reads of owner->on

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Date:   Mon, 06 Dec 2021 15:15:40 -0000
From:   "tip-bot2 for Marco Elver" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Marco Elver <elver@...gle.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: locking/core] locking: Mark racy reads of owner->on_cpu

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     4cf75fd4a2545ca4deea992f929602c9fdbe8058
Gitweb:        https://git.kernel.org/tip/4cf75fd4a2545ca4deea992f929602c9fdbe8058
Author:        Marco Elver <elver@...gle.com>
AuthorDate:    Fri, 03 Dec 2021 15:59:35 +08:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Sat, 04 Dec 2021 10:56:25 +01:00

locking: Mark racy reads of owner->on_cpu

One of the more frequent data races reported by KCSAN is the racy read
in mutex_spin_on_owner(), which is usually reported as "race of unknown
origin" without showing the writer. This is due to the racing write
occurring in kernel/sched. Locally enabling KCSAN in kernel/sched shows:

 | write (marked) to 0xffff97f205079934 of 4 bytes by task 316 on cpu 6:
 |  finish_task                kernel/sched/core.c:4632 [inline]
 |  finish_task_switch         kernel/sched/core.c:4848
 |  context_switch             kernel/sched/core.c:4975 [inline]
 |  __schedule                 kernel/sched/core.c:6253
 |  schedule                   kernel/sched/core.c:6326
 |  schedule_preempt_disabled  kernel/sched/core.c:6385
 |  __mutex_lock_common        kernel/locking/mutex.c:680
 |  __mutex_lock               kernel/locking/mutex.c:740 [inline]
 |  __mutex_lock_slowpath      kernel/locking/mutex.c:1028
 |  mutex_lock                 kernel/locking/mutex.c:283
 |  tty_open_by_driver         drivers/tty/tty_io.c:2062 [inline]
 |  ...
 |
 | read to 0xffff97f205079934 of 4 bytes by task 322 on cpu 3:
 |  mutex_spin_on_owner        kernel/locking/mutex.c:370
 |  mutex_optimistic_spin      kernel/locking/mutex.c:480
 |  __mutex_lock_common        kernel/locking/mutex.c:610
 |  __mutex_lock               kernel/locking/mutex.c:740 [inline]
 |  __mutex_lock_slowpath      kernel/locking/mutex.c:1028
 |  mutex_lock                 kernel/locking/mutex.c:283
 |  tty_open_by_driver         drivers/tty/tty_io.c:2062 [inline]
 |  ...
 |
 | value changed: 0x00000001 -> 0x00000000

This race is clearly intentional, and the potential for miscompilation
is slim due to surrounding barrier() and cpu_relax(), and the value
being used as a boolean.

Nevertheless, marking this reader would more clearly denote intent and
make it obvious that concurrency is expected. Use READ_ONCE() to avoid
having to reason about compiler optimizations now and in future.

With previous refactor, mark the read to owner->on_cpu in owner_on_cpu(),
which immediately precedes the loop executing mutex_spin_on_owner().

Signed-off-by: Marco Elver <elver@...gle.com>
Signed-off-by: Kefeng Wang <wangkefeng.wang@...wei.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lore.kernel.org/r/20211203075935.136808-3-wangkefeng.wang@huawei.com
---
 include/linux/sched.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index ff609d9..0b9b0e3 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2177,7 +2177,7 @@ static inline bool owner_on_cpu(struct task_struct *owner)
 	 * As lock holder preemption issue, we both skip spinning if
 	 * task is not on cpu or its cpu is preempted
 	 */
-	return owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
+	return READ_ONCE(owner->on_cpu) && !vcpu_is_preempted(task_cpu(owner));
 }
 
 /* Returns effective CPU energy utilization, as seen by the scheduler */