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

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Date:	Wed, 4 May 2011 22:49:36 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Valdis.Kletnieks@...edu,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tejun Heo <tj@...nel.org>, 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

* Christoph Lameter <cl@...ux.com> wrote:

> > 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) [...]

6 cycles for CMPXCHG8B versus 19 cycles for LOCK CMPXCHG8B, on Nehalem.

And note that it's not really true that the LOCK prefix-less CMPXCHG8B is not 
atomic: the write is atomic if the target word is naturally aligned, on i586 
and upwards ...

So in practice, unless the SLUB variables are misaligned, the lock prefix-less 
CMPXCHG8B *IS* atomic.

Non-atomicity is a special case of a weird special case.

> [...] is used in a unique way here in a percpu fastdpath. Thats why we have 
> the strange naming this_cpu_cmpxchg_double etc.

Furthermore, we never used cmpxchg in the kernel without expecting atomicity.

Uniquely strange, unintuitive naming == invitation for strange bugs.

And guess what, we had a strange bug here. Can you possibly see any connection?

Thanks,

	Ingo
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