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

[Ingo Molnar]

* Christoph Lameter <cl@xxxxxxxxx> 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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