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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jan 06, 2016 at 01:23:50PM -0500, Rich Felker wrote:
> On Wed, Jan 06, 2016 at 03:32:18PM +0100, Peter Zijlstra wrote:
> > On Wed, Jan 06, 2016 at 01:52:17PM +0200, Michael S. Tsirkin wrote:
> > > > > Peter, what do you think? How about I leave this patch as is for now?
> > > > 
> > > > No, and I object to removing the single byte implementation too. Either
> > > > remove the full arch or fix xchg() to conform. xchg() should work on all
> > > > native word sizes, for SH that would be 1,2 and 4 bytes.
> > > 
> > > Rick, maybe you could explain how is current 1 byte xchg on llsc wrong?
> > 
> > It doesn't seem to preserve the 3 other bytes in the word.
> > 
> > > It does use 4 byte accesses but IIUC that is all that exists on
> > > this architecture.
> > 
> > Right, that's not a problem, look at arch/alpha/include/asm/xchg.h for
> > example. A store to another portion of the word should make the
> > store-conditional fail and we'll retry the loop.
> > 
> > The short versions should however preserve the other bytes in the word.
> 
> Indeed. Also, accesses must be aligned, so the asm needs to round down
> to an aligned address and perform a correct read-modify-write on it,
> placing the new byte in the correct offset in the word.
> 
> Alternatively (my preference) this logic can be impemented in C as a
> wrapper around the 32-bit cmpxchg. I think this is less error-prone
> and it can be shared between the multiple sh cmpxchg back-ends,
> including the new cas.l one we need for J2.

Sounds much more reasonable.

> > SH's cmpxchg() is equally incomplete and does not provide 1 and 2 byte
> > versions.
> > 
> > In any case, I'm all for rm -rf arch/sh/, one less arch to worry about
> > is always good, but ISTR some people wanting to resurrect SH:
> > 
> >   http://old.lwn.net/Articles/647636/
> > 
> > Rob, Jeff, Sato-san, might I suggest you send a MAINTAINERS patch and
> > take up an active interest in SH lest someone 'accidentally' nukes it?
> 
> We're in the process of preparing such a proposal right now. That
> current intent is that Sato-san and I will co-maintain arch/sh. We'll
> include more details about motivation, proposed development direction,
> existing work to be merged, etc. in that proposal.
> 
> Rich