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

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Date:	Tue, 24 Jul 2007 13:42:12 -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 10:24 -0700, Linus Torvalds wrote:
> 
> 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.

That's not what the Documentation/memory-barriers.txt states:

        Any atomic operation that modifies some state in memory and returns information
        about the state (old or new) implies an SMP-conditional general memory barrier
        (smp_mb()) on each side of the actual operation.  These include:
        
        .....
        
        test_and_set_bit();
        test_and_clear_bit();
        test_and_change_bit();
...

Trond

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