Messages in this thread | | | Date | Thu, 2 Dec 2021 10:46:52 -0500 | Subject | Re: [PATCH] locking/mutex: Mark racy reads of owner->on_cpu | From | Waiman Long <> |
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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
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