Messages in this thread | | | Date | Thu, 2 Dec 2021 15:46:22 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] locking/mutex: Mark racy reads of owner->on_cpu |
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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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