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

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Date:	Tue, 24 Jul 2007 17:15:00 +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:

> > > 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.
> 
> The system bus does not need to be serialised because the CPU already
> holds the cacheline in exclusive state. That *is* the cache coherency
> protocol.
> 
> The memory ordering is enforced by the CPU effectively preventing
> speculative loads to pass the locked instruction and ensuring all
> stores reach the cache before it is executed. (I say effectively
> because the CPU might do clever tricks without you knowing).

Looks like when you said "CPU memory barrier extends to all memory
references" you were probably referring to a _given_ CPU ... yes,
that statement is correct in that case.

> > > 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.
> 
> Why?

Well, why not. Callers who don't want/need any guarantees whatsoever
can still use __foo() -- for others, it makes sense to just use
foo() and get *both* the compiler and CPU barrier semantics -- I think
that's the behaviour most callers would want anyway.


Satyam
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