On 7/17/19 3:22 PM, Jan Stancek wrote:
> On Wed, Jul 17, 2019 at 10:19:04AM -0400, Waiman Long wrote:
>>> If you add a comment to the code outlining the issue (preferably as
>>> a litmus
>>> test involving sem->count and some shared data which happens to be
>>> vmacache_seqnum in your test)), then:
>>>
>>> Reviewed-by: Will Deacon <w...@kernel.org>
>>>
>>> Thanks,
>>>
>>> Will
>>
>> Agreed. A comment just above smp_acquire__after_ctrl_dep() on why this
>> is needed will be great.
>>
>> Other than that,
>>
>> Acked-by: Waiman Long <long...@redhat.com>
>>
>
> litmus test looks a bit long, would following be acceptable?
>
> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> index 37524a47f002..d9c96651bfc7 100644
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -1032,6 +1032,13 @@ static inline bool
> rwsem_reader_phase_trylock(struct rw_semaphore *sem,
> */
> if (adjustment && !(atomic_long_read(&sem->count) &
> (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
> + /*
> + * down_read() issued ACQUIRE on enter, but we can race
> + * with writer who did RELEASE only after us.
> + * ACQUIRE here makes sure reader operations happen only
> + * after all writer ones.
> + */
How about that?
/*
* Add an acquire barrier here to make sure no stale data
* acquired before the above test where the writer may still
* be holding the lock will be reused in the reader
critical
* section.
*/
Thanks,
Longman
