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

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Date:	Tue, 24 Jul 2007 10:24:10 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
cc:	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, Benjamin Herrenschmidt wrote:
> 
> In fact, it's more than that... the bitops that return a value are often
> used to have hand-made spinlock semantics. I'm sure we would get funky
> bugs if loads or stores leaked out of the locked region. I think a full
> "memory" clobber should be kept around for those cases.

Not helpful.

The CPU ordering constraints for "test_and_set_bit()" and friends are weak 
enough that even if you have a full memory clobber, it still wouldn't work 
as a lock.

That's exactly why we have smp_mb__after_set_bit() and friends. On some 
architectures (arm, mips, parsic, powerpc), *that* is where the CPU memory 
barrier is, because the "test_and_set_bit()" itself is just a 
cache-coherent operation, not an actual barrier.

		Linus
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