lkml.org 
[lkml]   [2021]   [Dec]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH] locking/mutex: Mark racy reads of owner->on_cpu
From
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

\
 
 \ /
  Last update: 2021-12-02 16:47    [W:0.049 / U:1.104 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site