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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 4 May 2011 14:19:31 -0700
Message-ID: <BANLkTimvxOMuPJ0W0npo5i3gwFqdRw74Pw@mail.gmail.com>
Subject: Re: [block IO crash] Re: 2.6.39-rc5-git2 boot crashs
To: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        Tejun Heo <tj@kernel.org>, Ingo Molnar <mingo@elte.hu>,
        Jens Axboe <axboe@kernel.dk>,
        Andrew Morton <akpm@linux-foundation.org>, werner <w.landgraf@ru.ru>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Wed, May 4, 2011 at 2:06 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> The problem is that cmpxchg *IS*NOT*AN*CMPXCHG*AT*ALL*. Not even on UP.

Btw, the really sad thing is that as far as I can tell, the
cmpxchg64_local() function that we define actually gets this *right*,
with a helper function that actually works (and disables interrupts)
and all the alternative_io() stuff to then use the right instruction
automatically if the CPU supports it, rather than making it a
hardcoded decision.

But we don't use it, because the "percpu_cmpxchg8b_double()" code
rolls its own inferior version, and has slightly different semantics
(ie the whole "return value success" thing).

So close, but yet so far.

                       Linus
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