Re: [PATCH v2 31/32] sh: support a 2-byte smp_store_mb

[Michael S. Tsirkin]

On Tue, Jan 05, 2016 at 06:27:35PM -0500, Rich Felker wrote:
> On Thu, Dec 31, 2015 at 09:09:47PM +0200, Michael S. Tsirkin wrote:
> > At the moment, xchg on sh only supports 4 and 1 byte values, so using it
> > from smp_store_mb means attempts to store a 2 byte value using this
> > macro fail.
> > 
> > And happens to be exactly what virtio drivers want to do.
> > 
> > Check size and fall back to a slower, but safe, WRITE_ONCE+smp_mb.
> 
> Can you please do this for size 1 as well (i.e. all sizes != 4)? If
> you check the source, the code for size-1 xchg in sh cmpxchg-llsc.h is
> completely wrong and operates on a 32-bit object at the address passed
> to it. This code is presently unused anyway and I plan to submit a
> patch to remove the size 1 case.
> 
> Rich

Ouch. And PeterZ says I should write a 2-byte xchg in asm instead,
and Fedora can't even build a full kernel for this arch at the moment :(

Peter, what do you think? How about I leave this patch as is for now?

-- 
MST
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