linux-kernel - Re: [PATCH 6/8] i386: bitops: Don't mark memory as clobbered unnecessarily

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Message-Id: <1185301733.6586.38.camel@localhost>
Date:	Tue, 24 Jul 2007 14:28:53 -0400
From:	Trond Myklebust <trond.myklebust@....uio.no>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Satyam Sharma <ssatyam@....iitk.ac.in>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>,
	Nick Piggin <nickpiggin@...oo.com.au>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 6/8] i386: bitops: Don't mark memory as clobbered
	unnecessarily

On Tue, 2007-07-24 at 11:13 -0700, Linus Torvalds wrote:
> 
> On Tue, 24 Jul 2007, Trond Myklebust wrote:
> > 
> > That's not what the Documentation/memory-barriers.txt states:
> 
> Hmm.. You're right. We only actually need it for the unconditional bitops, 
> like the *unlock* side.
> 
> IOW, if you do a spinlock with the bitops, the locking side should be able 
> to use a "test_and_set_bit()" on its own, but the unlocking side should be
> 
> 	smp_mb__before_clear_bit();
> 	clear_bit();
> 
> because the ones that don't return a value also don't imply a memory 
> barrier.

Yup, and this is exactly what we currently do in bit_spin_unlock().

Trond

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