Re: [rfc][patch] i386: remove comment about barriers

From: Nick Piggin
Date: Sun Sep 30 2007 - 08:06:05 EST

On Sat, Sep 29, 2007 at 12:12:52PM -0700, Davide Libenzi wrote:
> On Sat, 29 Sep 2007, Nick Piggin wrote:
> > [ This is true for x86's sfence/lfence, but raises a question about Linux's
> > memory barriers. Does anybody expect that a sequence of smp_wmb and smp_rmb
> > would ever provide a full smp_mb barrier? I've always assumed no, but I
> > don't know if it is actually documented? ]
> No, that can't be. rmb+wmb can't be considered a full mb.

Oh I realise it doesn't work with current implementations (powerpc at
least also would be broken I think). And I've always assumed no... But
the question was just whether the Linux memory model requires it (it
theoretically could require that smp_rmb is sequentially ordered WRT
smp_wmb without actually sequentially ordering stores around itself).

> There was a
> recent discussion about this in the thread originated by peterz scalable
> rw_mutex patches.

OK, good to hear it has been discussed. I didn't see anything explicit
in the documentation about it. A lot of the literature often says that
"barrier instructions" have a sequential ordering, so it could be useful
to explicitly say this isn't the case for Linux for rmb vs wmb.

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