linux-kernel - Re: [block IO crash] Re: 2.6.39-rc5-git2 boot crashs

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Date:	Wed, 4 May 2011 11:20:22 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Pekka Enberg <penberg@...nel.org>
Cc:	Christoph Lameter <cl@...ux.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	Jens Axboe <axboe@...nel.dk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	werner <w.landgraf@...ru>, "H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [block IO crash] Re: 2.6.39-rc5-git2 boot crashs

On Wed, May 4, 2011 at 8:53 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> I'll take it. I'm just hoping to also get Werner's tested-by for it.
>
> I'm pretty confident this is it, though, so if I don't get it by the
> end of the day I'll just apply it regardless with just a reported-by
> from him.

So I'm still waiting for the tested-by, but in the meantime I wrote a
changelog. And part of that changelog reads:

[ Btw, that whole "generic code defaults to no protection" design just
  sounds stupid - if the code needs no protection, there is no reason to
  use "cmpxchg_double" to begin with.  So we should probably just remove
  the unprotected version entirely as pointless.   - Linus ]

which really sums up the whole thing.

The current "this_cpu_cmpxchg_double()" implementation is just
incredibly idiotic. There is absolutely _no_ point to having that
function at all. Why does it exist?

I can kind of see the point of the "preempt" version, although I'm not
entirely convinced of that either. But the notion of having a
"cmpxchg" function that isn't atomic even on a single CPU just makes
me go "f*ck that, whoever wrote that is just a moron".

If the function doesn't need atomicity, you're really better off just
writing it out.  It's going to be faster on pretty much all
architectures using just regular load/store instructions.

                   Linus
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