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Date: Tue, 28 Apr 2015 18:24:58 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Chris Metcalf <cmetcalf@ezchip.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Manfred Spraul <manfred@colorfullife.com>,
        Oleg Nesterov <oleg@redhat.com>, Kirill Tkhai <ktkhai@parallels.com>,
        linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking
 cycles
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On Tue, Apr 28, 2015 at 11:53:21AM -0400, Chris Metcalf wrote:
> As you surmise, tilepro doesn't have 64-bit loads.  So we are stuck with
> 32-bit loads on these two fields.  It's true that spin_unlock_wait() can
> therefore falsely claim that the lock is unlocked, but it should be only a
> hint anyway, since by the time the caller tries to act on that information
> the lock may have been retaken anyway, right?  If spin_unlock_wait() is
> really trying to guarantee that the lock was available at some point in
> the interval between when it was called and when it returned, we could use
> READ_ONCE() to read the current ticket value first; is that a necessary
> part of the semantics?

I think it must not return before the lock holder that is current at the
time of calling releases. Anything thereafter is indeed fair game as per
your logic above.