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SubjectRe: [PATCH] locking/mutex: Mark racy reads of owner->on_cpu
On Thu, Dec 02, 2021 at 12:53:14PM +0100, Marco Elver wrote:
> On Thu, 2 Dec 2021 at 11:13, Marco Elver <elver@google.com> wrote:
> > 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.
> >
> > Similarly, mark the read to owner->on_cpu in mutex_can_spin_on_owner(),
> > which immediately precedes the loop executing mutex_spin_on_owner().
> >
> > Signed-off-by: Marco Elver <elver@google.com>
> [...]
>
> Kefeng kindly pointed out that there is an alternative, which would
> refactor owner_on_cpu() from rwsem that would address both mutex and
> rwsem:
> https://lore.kernel.org/all/b641f1ea-6def-0fe4-d273-03c35c4aa7d6@huawei.com/

That seems to make sense, except it should probably go under CONFIG_SMP,
since ->on_cpu doesn't otherwise exist.

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