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

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Date:	Tue, 24 Jul 2007 20:25:26 +1000 (EST)
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Satyam Sharma <ssatyam@....iitk.ac.in>
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

--- Satyam Sharma <ssatyam@....iitk.ac.in> wrote:

> 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.

 
> In any case, I know that, obviously. I asked "why"
> not "what" :-) i.e.
> why should we care about other addresses / why do we
> want to extend
> the compiler barrier to all memory references -- but
> Jeremy seems to
> have answered that ...

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.


> > Repeating what has been said before: A CPU memory
> barrier is not a
> > compiler barrier or vice versa. Seeing as we are
> talking about
> > the compiler barrier, it is irrelevant as to
> whether or not the
> > assembly includes a CPU barrier.
> 
> I think it is quite relevant, in fact. From
> Documentation/atomic_ops.txt,
> smp_mb__{before,after}_clear_bit(), as the name
> itself suggests, must
> be _CPU barriers_ for those arch's that don't have
> an implicit
> _CPU barrier_ in the clear_bit() itself [ which i386
> does have already ].
> 
> 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.

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);

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?



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