linux-kernel - Re: [PATCH v5 tip/core/locking 5/7] Documentation/memory-barriers.txt: Downgrade UNLOCK+LOCK

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Date:	Tue, 10 Dec 2013 18:35:24 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, darren@...art.com, fweisbec@...il.com,
	sbw@....edu, Ingo Molnar <mingo@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Will Deacon <will.deacon@....com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Waiman Long <waiman.long@...com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	"H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH v5 tip/core/locking 5/7]
	Documentation/memory-barriers.txt: Downgrade UNLOCK+LOCK

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.

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