Date: Wed, 4 May 2011 15:32:55 -0500 (CDT)
From: Christoph Lameter <cl@linux.com>
To: Valdis.Kletnieks@vt.edu
cc: Linus Torvalds <torvalds@linux-foundation.org>,
        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>
Subject: Re: [block IO crash] Re: 2.6.39-rc5-git2 boot crashs
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On Wed, 4 May 2011, Valdis.Kletnieks@vt.edu wrote:

> On Wed, 04 May 2011 15:04:39 CDT, Christoph Lameter said:
> > On Wed, 4 May 2011, Linus Torvalds wrote:
>
> > > But cmpxchg DOES NOT MAKE SENSE without atomicity guarantees.
> >
> > This is not a real cmpxchg after all. Its not atomic in the sense of
> > other functions. Its only "percpu atomic" if you want it that way. This is
> > *not* a full cmpxchg_double().
>
> Calling it a cmpxchg when it doesn't have the primary distinguishing property
> of a hardware cmpxchg is just loading a bullet in the chamber and inviting
> kernel hackers to point it at their feet...

It does have most of the distinguishing characterstics but the
lock-prefixless cmpxchg8b/16b (which is quite fast) is used in a
unique way here in a percpu fastdpath. Thats why we have the strange
naming this_cpu_cmpxchg_double etc.


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