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@xxxxxxxxxx> AuthorDate: Fri, 03 Dec 2021 15:59:35 +08:00 Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxx> Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Link: https://lore.kernel.org/r/20211203075935.136808-3-wangkefeng.wang@xxxxxxxxxx --- 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 */
|