linux-kernel - Re: [PATCH v2 31/32] sh: support a 2-byte smp_store

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Date:	Thu, 7 Jan 2016 23:25:05 -0500
From:	Rich Felker <dalias@...c.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, linux-sh@...r.kernel.org,
	Rob Landley <rob@...dley.net>, Jeff Dionne <jeff@...inux.org>,
	Yoshinori Sato <ysato@...rs.sourceforge.jp>
Subject: Re: [PATCH v2 31/32] sh: support a 2-byte smp_store_mb

On Fri, Jan 08, 2016 at 12:41:35AM +0200, Michael S. Tsirkin wrote:
> > > > 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.
> > 
> > In principle I agree, but u8/u16 xchg is mostly unused (completely
> > unused in my builds) and unlikely to matter to performance. Also, the
> > irq variant is only for the original sh2 which is not even produced
> > anymore afaik. Our reimplementation of the sh2 ISA, the J2, has a
> > cas.l instruction that will be used instead because it supports SMP
> > where interrupt masking is insufficient to achieve atomicity.
> > 
> > Rich
> 
> Since it looks like there will soon be active maintainers
> for this arch, I think it's best if I make the minimal possible
> changes and then you guys can rewrite it any way you like,
> drop irq variant or whatever.
> 
> The minimal change is probably the below code but
> the grb variant is just copy paste from xchg_u8
> with a minor tweak -
> can you pls confirm it looks right?

I haven't had a chance to test it, but I don't see anything obviously
wrong with it.

> I tested the llsc code on ppc and x86 and since it's
> portable I know the logic is correct there.

Sounds good. Since it will also be needed for the cas.l variant I'd
rather have this in the main asm/cmpxchg.h where it can be shared if
you see an easy way to do that now, but if not I can take care of it
later when merging cmpxchg-cas.h. Perhaps just putting __xchg_cmpxchg
in the main asm/cmpxchg.h would suffice so that only the thin wrappers
need to be duplicated. Ideally it could even be moved outside of the
arch asm headers, but then there might be annoying header ordering
issues to deal with.

> Will post v3 with this included but would appreciate
> your input first.

Go ahead. Thanks!

Rich