From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <waiman.long@hpe.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>,
manfred@colorfullife.com, mingo@kernel.org,
torvalds@linux-foundation.org, ggherdovich@suse.com,
mgorman@techsingularity.net, linux-kernel@vger.kernel.org,
Paul McKenney <paulmck@linux.vnet.ibm.com>,
Will Deacon <will.deacon@arm.com>
Subject: Re: sem_lock() vs qspinlocks
Date: Fri, 20 May 2016 22:53:00 +0200 [thread overview]
Message-ID: <20160520205300.GJ3193@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <573F7723.8030201@hpe.com>
On Fri, May 20, 2016 at 04:44:19PM -0400, Waiman Long wrote:
> On 05/20/2016 07:58 AM, Peter Zijlstra wrote:
> >On Thu, May 19, 2016 at 10:39:26PM -0700, Davidlohr Bueso wrote:
> >>As such, the following restores the behavior of the ticket locks and 'fixes'
> >>(or hides?) the bug in sems. Naturally incorrect approach:
> >>
> >>@@ -290,7 +290,8 @@ static void sem_wait_array(struct sem_array *sma)
> >>
> >> for (i = 0; i< sma->sem_nsems; i++) {
> >> sem = sma->sem_base + i;
> >>- spin_unlock_wait(&sem->lock);
> >>+ while (atomic_read(&sem->lock))
> >>+ cpu_relax();
> >> }
> >> ipc_smp_acquire__after_spin_is_unlocked();
> >>}
> >The actual bug is clear_pending_set_locked() not having acquire
> >semantics. And the above 'fixes' things because it will observe the old
> >pending bit or the locked bit, so it doesn't matter if the store
> >flipping them is delayed.
>
> The clear_pending_set_locked() is not the only place where the lock is set.
> If there are more than one waiter, the queuing patch will be used instead.
> The set_locked(), which is also an unordered store, will then be used to set
> the lock.
Ah yes. I didn't get that far. One case was enough :-)
prev parent reply other threads:[~2016-05-20 20:53 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-05-20 5:39 Davidlohr Bueso
2016-05-20 7:49 ` Peter Zijlstra
2016-05-20 15:00 ` Davidlohr Bueso
2016-05-20 15:05 ` Peter Zijlstra
2016-05-20 15:25 ` Davidlohr Bueso
2016-05-20 15:28 ` Peter Zijlstra
2016-05-20 20:47 ` Waiman Long
2016-05-20 20:52 ` Peter Zijlstra
2016-05-21 0:59 ` Davidlohr Bueso
2016-05-21 4:01 ` Waiman Long
2016-05-21 7:40 ` Peter Zijlstra
2016-05-20 7:53 ` Peter Zijlstra
2016-05-20 8:13 ` Peter Zijlstra
2016-05-20 8:18 ` Peter Zijlstra
2016-05-20 9:07 ` Giovanni Gherdovich
2016-05-20 9:34 ` Peter Zijlstra
2016-05-20 8:30 ` Peter Zijlstra
2016-05-20 9:00 ` Peter Zijlstra
2016-05-20 10:09 ` Ingo Molnar
2016-05-20 10:45 ` Mel Gorman
2016-05-20 11:58 ` Peter Zijlstra
2016-05-20 14:05 ` Boqun Feng
2016-05-20 15:21 ` Peter Zijlstra
2016-05-20 16:04 ` Peter Zijlstra
2016-05-20 17:00 ` Linus Torvalds
2016-05-20 21:06 ` Peter Zijlstra
2016-05-20 21:44 ` Linus Torvalds
2016-05-21 0:48 ` Davidlohr Bueso
2016-05-21 2:30 ` Linus Torvalds
2016-05-21 7:37 ` Peter Zijlstra
2016-05-21 13:49 ` Manfred Spraul
2016-05-24 10:57 ` Peter Zijlstra
2016-05-21 17:14 ` Davidlohr Bueso
2016-05-23 12:25 ` Peter Zijlstra
2016-05-23 17:52 ` Linus Torvalds
2016-05-25 6:37 ` Boqun Feng
2016-05-22 8:43 ` Manfred Spraul
2016-05-22 9:38 ` Peter Zijlstra
2016-05-20 16:20 ` Davidlohr Bueso
2016-05-20 20:44 ` Waiman Long
2016-05-20 20:53 ` Peter Zijlstra [this message]
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