From: Peter Zijlstra <peterz@infradead.org>
To: Peter Hurley <peter@hurleysoftware.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>,
mingo@kernel.org, tglx@linutronix.de, Waiman.Long@hpe.com,
jason.low2@hp.com, linux-kernel@vger.kernel.org,
Davidlohr Bueso <dbueso@suse.de>
Subject: Re: [PATCH 1/4] locking/rwsem: Avoid stale ->count for rwsem_down_write_failed()
Date: Mon, 9 May 2016 09:12:20 +0200 [thread overview]
Message-ID: <20160509071220.GB3430@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <573021D5.4070406@hurleysoftware.com>
On Sun, May 08, 2016 at 10:36:21PM -0700, Peter Hurley wrote:
> On 05/08/2016 09:56 PM, Davidlohr Bueso wrote:
> > The field is obviously updated w.o the lock and needs a READ_ONCE
> > while waiting for lock holder(s) to go away, just like we do with
> > all other ->count accesses.
>
> This isn't actually fixing a bug because it's passed through
> several full barriers which will force reloading from sem->count.
>
> I think the patch is ok if you want it just for consistency anyway,
> but please change $subject and changelog.
Agreed, and note that the READ_ONCE does prohibit load-tearing, while
the current code does not.
So in that respect the patched code is more strict. But yes, the current
code does not allow using a stale value of sem->count() because, as
PeterH notes, we've just passed through at least the full memory barrier
implied by set_current_state().
next prev parent reply other threads:[~2016-05-09 7:12 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-05-09 4:56 [PATCH -tip 0/4] locking/rwsem (xadd): Reader waiter optimizations Davidlohr Bueso
2016-05-09 4:56 ` [PATCH 1/4] locking/rwsem: Avoid stale ->count for rwsem_down_write_failed() Davidlohr Bueso
2016-05-09 5:36 ` Peter Hurley
2016-05-09 7:12 ` Peter Zijlstra [this message]
2016-05-09 14:29 ` Davidlohr Bueso
2016-05-09 4:56 ` [PATCH 2/4] locking/rwsem: Drop superfluous waiter refcount Davidlohr Bueso
2016-05-09 7:39 ` Peter Zijlstra
2016-05-09 15:56 ` Davidlohr Bueso
2016-05-09 16:11 ` Peter Zijlstra
2016-05-09 18:51 ` Davidlohr Bueso
2016-05-09 18:59 ` Peter Zijlstra
2016-05-09 4:56 ` [PATCH 3/4] locking/rwsem: Enable lockless waiter wakeup(s) Davidlohr Bueso
2016-05-09 7:48 ` Peter Zijlstra
2016-05-09 4:56 ` [PATCH 4/4] locking/rwsem: Rework zeroing reader waiter->task Davidlohr Bueso
2016-05-09 7:50 ` Peter Zijlstra
2016-05-10 1:31 ` [PATCH -tip 0/4] locking/rwsem (xadd): Reader waiter optimizations Waiman Long
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