linux-kernel - Re: [PATCH 8/8] i386: bitops: smp_mb__{before, after}_clear

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Date:	Tue, 24 Jul 2007 16:40:09 +0530 (IST)
From:	Satyam Sharma <ssatyam@....iitk.ac.in>
To:	Nick Piggin <nickpiggin@...oo.com.au>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 8/8] i386: bitops: smp_mb__{before, after}_clear_bit()
 definitions

On Tue, 24 Jul 2007, Nick Piggin wrote:

> > > Satyam Sharma wrote:
> 
> > > > Consider this (the above two functions exist
> > only for clear_bit(),
> > > > the atomic variant, as you already know), the
> > _only_ memory reference
> > > > we care about is that of the address of the
> > passed bit-string:
> > > 
> > > No. Memory barriers explicitly extend to all
> > memory references.
> > 
> > [ Compiler barrier, you mean, that's not true of CPU
> > barriers. ]
> 
> For the purpose of this discussion (Linux memory
> barrier semantics, on WB memory), it is true of CPU
> and compiler barriers.

On later Intel processors, if the memory address range being referenced
(and say written to) by the (locked) instruction is in the cache of a
CPU, then it will not assert the LOCK signal on the system bus --
thus not assume exclusive use of shared memory. So other CPUs are free
to modify (other) memory at that point. Cache coherency will still
ensure _that_ (locked) memory area is still updated atomically, though.

> Obviously because we want some kind of ordering
> guarantee at a given point. All the CPU barriers
> in the world are useless if the compiler can reorder
> access over them.

Yes ...

> > As for a compiler barrier, the asm there already
> > guarantees the compiler
> > will not optimize references to _that_ address
> 
> One or both of us still fails to understand the other.

... I think the culprit is me ...

> bit_spin_lock(LOCK_NR, &word);
> var++;
> /* this is bit_spin_unlock(LOCK_NR, &word); */
> smp_mb__before_clear_bit();
> clear_bit(LOCK_NR, &word);

Yup, David has laid this out clearly, as well.

> Are you saying that it is OK for the store to var to
> be reordered below the clear_bit? If not, what are you
> saying?

I might be making a radical turn-around here, but all of a
sudden I think it's actually a good idea to put a complete
memory clobber in set_bit/clear_bit and friends themselves,
and leave the "__" variants as they are.


Satyam
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