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Date: Tue, 10 Dec 2013 18:35:24 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.com,
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        Benjamin Herrenschmidt <benh@kernel.crashing.org>
Subject: Re: [PATCH v5 tip/core/locking 5/7]
	Documentation/memory-barriers.txt: Downgrade UNLOCK+LOCK
Message-ID: <20131210173524.GB25934@redhat.com>
References: <20131210012738.GA24317@linux.vnet.ibm.com> <1386638883-25379-1-git-send-email-paulmck@linux.vnet.ibm.com> <1386638883-25379-5-git-send-email-paulmck@linux.vnet.ibm.com> <20131210164436.GA23506@redhat.com> <20131210171522.GR4208@linux.vnet.ibm.com>
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On 12/10, Paul E. McKenney wrote:
>
> On Tue, Dec 10, 2013 at 05:44:37PM +0100, Oleg Nesterov wrote:
> >
> > Well, but smp_mb__before_spinlock + LOCK is not wmb... But it is not
> > the full barrier. It should guarantee that, say,
> >
> > 	CONDITION = true;		// 1
> >
> > 	// try_to_wake_up
> > 	smp_mb__before_spinlock();
> > 	spin_lock(&task->pi_lock);
> >
> > 	if (!(p->state & state))	// 2
> > 		return;
> >
> > can't race with with set_current_state() + check(CONDITION), this means
> > that 1 and 2 above must not be reordered.
> >
> > But a LOAD before before spin_lock() can leak into the critical section.
> >
> > Perhaps this should be clarified somehow, or perhaps it should actually
> > imply mb (if combined with LOCK).
>
> If we leave the implementation the same, does the following capture the
> constraints?
>
> 	Memory operations issued before the LOCK may be completed after
> 	the LOCK operation has completed.  An smp_mb__before_spinlock(),
> 	combined with a following LOCK, orders prior loads against
> 	subsequent stores

prior stores against subsequent loads ;)

Otherwise - thanks!

Oleg.