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

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Message-ID: <alpine.DEB.2.00.1105041530210.5495@router.home>
Date:	Wed, 4 May 2011 15:32:55 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Valdis.Kletnieks@...edu
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	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, 4 May 2011, Valdis.Kletnieks@...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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