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

[Thomas Gleixner]

On Wed, 4 May 2011, Tejun Heo wrote:

> Hello,
> 
> On Wed, May 04, 2011 at 04:10:29PM +0200, Thomas Gleixner wrote:
> > > cases where the generic implementation is used.  Has anyone measured
> > > the difference against before the whole this_cpu conversion?
> > 
> > Yes, that really wants to be done. The whole CMPXCHG_LOCAL ifdeffery
> > should have been avoided in the first place. this_cpu_cmpxchg can
> > really be implemented with preempt_enable/disable and the irqsafe
> > variant in any case.
> 
> Yeah, slub code looks pretty scary with the #ifdefs.  IIUC, the
> problem was that cmpxchg_double is an optimization for fast path which
> was already very light weight and an extra locked op or irq on/off
> would have made considerable difference.
> 
> The cmpxchg_double optimization made the fast path go quite faster
> when CPU supports it but it may as well slow things down considerably
> if CPU doesn't, due to extra irq on/off's.  Anyways, here's hoping

Not really.

CMPXCHG_LOCAL=n

	local_irq_save();
	  handle_everything();
	local_irq_restore();

vs.

CMPXCHG_LOCAL=y

	do_prep();
	local_irq_save();
	  emulate_cmpxchg();
	local_irq_restore();
	do_rest();

So you have local irq disable/enable in both cases. So for the case
where you don't have a local cmpxchg8b/16b available it's not worse
versus irq disable/enable than now. It just has the possible repeat
case when stuff changed between the prep and the actual cmpxchg, which
is the same problem when cmpxchg8b/16 is available.

Thanks,

	tglx



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