Date: Wed, 4 May 2011 23:40:33 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
cc: Christoph Lameter <cl@linux.com>, Pekka Enberg <penberg@kernel.org>,
        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>
Subject: Re: [block IO crash] Re: 2.6.39-rc5-git2 boot crashs
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On Wed, 4 May 2011, Linus Torvalds wrote:

> 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).

Yeah, I tripped over that as well. And the new shiny extra
CONFIG_CMPXCHG_DOUBLE misfeature Christoph nailed together is just
ignoring it as well. It's just more useless #ifdef and CONFIG bloat.

That whole this_cpu_* stuff seems to be designed by committee. A quick
grep shows that max. 10% of the implemented macro hell is actually
used. Can we get rid of all unused ones and bring them back when they
are actually required?

Thanks,

	tglx
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