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 06:53:01PM -0500, Rich Felker wrote:
> On Wed, Jan 06, 2016 at 10:23:12PM +0200, Michael S. Tsirkin wrote:
> > 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.
> > > 
> > > > 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.
> > 
> > Well I'd like to be able to make progress with generic
> > arch cleanups meanwhile.
> > 
> > Could you quickly write a version of 1 and 2 byte xchg that
> > works so I can include it?
> 
> Here are quick, untested generic ones:
> 
> static inline unsigned long xchg_u8(volatile u8 *m, unsigned long val)
> {
> 	u32 old;
> 	unsigned long offset = (unsigned long)m & 3;
> 	volatile u32 *w = (volatile u32 *)(m - offset);
> 	union { u32 w; u8 b[4]; } u;
> 	do {
> 		old = u.w = *w;
> 		result = w.b[offset];
> 		w.b[offset] = val;
> 	} while (cmpxchg(w, old, u.w) != old);
> 	return result;
> }
> 
> static inline unsigned long xchg_u16(volatile u16 *m, unsigned long val)
> {
> 	u32 old;
> 	unsigned long result;
> 	unsigned long offset = ((unsigned long)m & 3) >> 1;
> 	volatile u32 *w = (volatile u32 *)(m - offset);
> 	union { u32 w; u16 h[2]; } u;
> 	do {
> 		old = u.w = *w;
> 		result = w.h[offset];
> 		w.h[offset] = val;
> 	} while (cmpxchg(w, old, u.w) != old);
> 	return result;
> }
> 
> It would be nice to have these in asm-generic for archs which don't
> define their own versions rather than having cruft like this repeated
> per-arch. Strictly speaking, the volatile u32 used to access the
> 32-bit word containing the u8 or u16 should be
> __attribute__((__may_alias__)) too. Is there an existing kernel type
> for a "may_alias u32" or should it perhaps be added?
> 
> Rich

BTW this seems suboptimal for grb and irq variants which apparently
can do things correctly.