linux-kernel - Re: [PATCH v5 tip/core/locking 6/7] locking: Add an smp_mb__after_unlock

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Date:	Mon, 9 Dec 2013 17:34:17 -0800
From:	Josh Triplett <josh@...htriplett.org>
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,
	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,
	Linux-Arch <linux-arch@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v5 tip/core/locking 6/7] locking: Add an
 smp_mb__after_unlock_lock() for UNLOCK+LOCK barrier

On Mon, Dec 09, 2013 at 05:28:02PM -0800, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> 
> The Linux kernel has traditionally required that an UNLOCK+LOCK pair
> act as a full memory barrier when either (1) that UNLOCK+LOCK pair
> was executed by the same CPU or task, or (2) the same lock variable
> was used for the UNLOCK and LOCK.  It now seems likely that very few
> places in the kernel rely on this full-memory-barrier semantic, and
> with the advent of queued locks, providing this semantic either requires
> complex reasoning, or for some architectures, added overhead.
> 
> This commit therefore adds a smp_mb__after_unlock_lock(), which may be
> placed after a LOCK primitive to restore the full-memory-barrier semantic.
> All definitions are currently no-ops, but will be upgraded for some
> architectures when queued locks arrive.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Linux-Arch <linux-arch@...r.kernel.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>

It seems quite unfortunate that this isn't in some common location, and
then only overridden by architectures that need to do so.

More importantly: you document this earlier in the patch series than you
introduce it.

- Josh Triplett
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