Re: [PATCH v2] locking/rwsem: add acquire barrier to read_slowpath exit when queue is empty

[Waiman Long]

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 <will@xxxxxxxxxx>
>>>
>>> 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 <longman@xxxxxxxxxx>
>>
>
> 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,
> ÂÂÂÂÂÂÂÂÂ */
> ÂÂÂÂÂÂÂÂ if (adjustment && !(atomic_long_read(&sem->count) &
> ÂÂÂÂÂÂÂÂÂÂÂÂÂ (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
> +ÂÂÂÂÂÂÂÂÂÂÂ /*
> +ÂÂÂÂÂÂÂÂÂÂÂÂ * down_read() issued ACQUIRE on enter, but we can race
> +ÂÂÂÂÂÂÂÂÂÂÂÂ * with writer who did RELEASE only after us.
> +ÂÂÂÂÂÂÂÂÂÂÂÂ * ACQUIRE here makes sure reader operations happen only
> +ÂÂÂÂÂÂÂÂÂÂÂÂ * after all writer ones.
> +ÂÂÂÂÂÂÂÂÂÂÂÂ */


How about that?

ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ /*
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * Add an acquire barrier here to make sure no stale data
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * acquired before the above test where the writer may still
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * be holding the lock will be reused in the reader
critical
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * section.
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ */

Thanks,
Longman