From: Jan Stancek <jstancek@redhat.com>
To: Waiman Long <longman@redhat.com>
Cc: Will Deacon <will@kernel.org>,
linux-kernel@vger.kernel.org, dbueso@suse.de,
peterz@infradead.org, mingo@redhat.com, jstancek@redhat.com
Subject: Re: [PATCH v2] locking/rwsem: add acquire barrier to read_slowpath exit when queue is empty
Date: Wed, 17 Jul 2019 21:22:00 +0200 [thread overview]
Message-ID: <20190717192200.GA17687@dustball.usersys.redhat.com> (raw)
In-Reply-To: <21ff5905-198b-6ea5-6c2a-9fb10cb48ea7@redhat.com>
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.
+ */
+ smp_acquire__after_ctrl_dep();
raw_spin_unlock_irq(&sem->wait_lock);
rwsem_set_reader_owned(sem);
lockevent_inc(rwsem_rlock_fast);
with litmus test in commit log:
----------------------------------- 8< ------------------------------------
C rwsem
{
atomic_t rwsem_count = ATOMIC_INIT(1);
int vmacache_seqnum = 10;
}
P0(int *vmacache_seqnum, atomic_t *rwsem_count)
{
r0 = READ_ONCE(*vmacache_seqnum);
WRITE_ONCE(*vmacache_seqnum, r0 + 1);
/* downgrade_write */
r1 = atomic_fetch_add_release(-1+256, rwsem_count);
}
P1(int *vmacache_seqnum, atomic_t *rwsem_count, spinlock_t *sem_wait_lock)
{
/* rwsem_read_trylock */
r0 = atomic_add_return_acquire(256, rwsem_count);
/* rwsem_down_read_slowpath */
spin_lock(sem_wait_lock);
r0 = atomic_read(rwsem_count);
if ((r0 & 1) == 0) {
// BUG: needs barrier
spin_unlock(sem_wait_lock);
r1 = READ_ONCE(*vmacache_seqnum);
}
}
exists (1:r1=10)
----------------------------------- 8< ------------------------------------
Thanks,
Jan
next prev parent reply other threads:[~2019-07-17 19:22 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 [this message]
2019-07-17 19:39 ` Waiman Long
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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