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 17:44:37 +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 12/09, Paul E. McKenney wrote:
>
> @@ -1626,7 +1626,10 @@ for each construct.  These operations all imply certain barriers:
>       operation has completed.
>
>       Memory operations issued before the LOCK may be completed after the LOCK
> -     operation has completed.
> +     operation has completed.  An smp_mb__before_spinlock(), combined
> +     with a following LOCK, acts as an smp_wmb().  Note the "w",
> +     this is smp_wmb(), not smp_mb().

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).

Oleg

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