Re: [PATCH] [SCSI] mpt2sas : Fix unsafe using smp_processor_id() in preemptible

[Matthew Wilcox <matthew@wil.cx>]

On Tue, Apr 17, 2012 at 09:35:49PM -0600, Moore, Eric wrote:
>  Jan Schmidt suggested using raw_smp_processor_id()  back in February, see this: http://marc.info/?t=132974687100003&r=1&w=2
> 
> Alex Shi recently suggested using preempt_disable() and preempt_enable() to solve the same issue, see this: http://marc.info/?t=133274303900003&r=1&w=2
> 
> I believe the stack traces are there in both email discussion, they occur when CONFIG_DEBUG_PREEMPT is enabled.
> 
> I would rather go with the solution giving the best performance.   James Bottomley is there on the discussion with  Jan Schmidt, he suggested using get_cpu() and put_cpu().

I use get_cpu() / put_cpu() in the NVMe driver in similar circumstances.
It's not noticable in the profiles :-)

Where my usage differs from the patch for mpt2sas is that I hold a
reference to the CPU over the submission.  This works out well for
me because I have per-CPU state.  Might be worth considering for your
driver ...

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."