From: Waiman Long <longman@redhat.com>
To: Jan Stancek <jstancek@redhat.com>
Cc: Will Deacon <will@kernel.org>,
linux-kernel@vger.kernel.org, dbueso@suse.de,
peterz@infradead.org, mingo@redhat.com
Subject: Re: [PATCH v2] locking/rwsem: add acquire barrier to read_slowpath exit when queue is empty
Date: Wed, 17 Jul 2019 15:39:37 -0400 [thread overview]
Message-ID: <1950f8bd-e0f4-9b65-fee6-701ecf531d1c@redhat.com> (raw)
In-Reply-To: <20190717192200.GA17687@dustball.usersys.redhat.com>
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@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 <longman@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
next prev parent reply other threads:[~2019-07-17 19:39 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-16 16:04 [PATCH] locking/rwsem: use read_acquire in read_slowpath exit when queue is empty Jan Stancek
2019-07-16 16:53 ` Waiman Long
2019-07-16 18:34 ` Jan Stancek
2019-07-16 18:58 ` Peter Zijlstra
2019-07-16 19:09 ` Waiman Long
2019-07-17 12:02 ` [PATCH v2] locking/rwsem: add acquire barrier to " Jan Stancek
2019-07-17 13:13 ` Will Deacon
2019-07-17 14:19 ` Waiman Long
2019-07-17 19:22 ` Jan Stancek
2019-07-17 19:39 ` Waiman Long [this message]
2019-07-18 8:51 ` [PATCH v3] " Jan Stancek
2019-07-25 16:00 ` [tip:locking/core] locking/rwsem: Add missing ACQUIRE " tip-bot for Jan Stancek
2019-07-18 9:26 ` [PATCH v2] locking/rwsem: add acquire barrier " Will Deacon
2019-07-18 10:50 ` Jan Stancek
2019-07-18 11:04 ` Peter Zijlstra
2019-07-18 11:09 ` Peter Zijlstra
2019-07-18 11:36 ` Jan Stancek
2019-07-18 12:12 ` Paul E. McKenney
2019-07-18 10:58 ` Peter Zijlstra
2019-07-18 11:45 ` Will Deacon
2019-07-18 12:23 ` Peter Zijlstra
2019-07-17 15:33 ` Waiman Long
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