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

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Date:	Tue, 24 Jul 2007 11:13:04 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Trond Myklebust <trond.myklebust@....uio.no>
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, 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.

		Linus
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