Re: [PATCH] locking/mutex: Mark racy reads of owner->on_cpu

[Waiman Long <longman@redhat.com>]

On 12/2/21 06:53, 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/
>
> Preferences?

I would like to see owner_on_cpu() extracted out from 
kernel/locking/rwsem.c into include/linux/sched.h right after 
vcpu_is_preempted(), for instance, and with READ_ONCE() added. Then it 
can be used in mutex.c as well. This problem is common to both mutex and 
rwsem.

Cheers,
Longman

Thanks,
-- Marco