Re: [tip:locking/core] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire

[Will Deacon <will@kernel.org>]

[+Palmer, PaulW, Daniel and Michael]

On Thu, Sep 09, 2021 at 09:25:30AM +0200, Peter Zijlstra wrote:
> On Wed, Sep 08, 2021 at 09:08:33AM -0700, Linus Torvalds wrote:
> 
> > So if this is purely a RISC-V thing,
> 
> Just to clarify, I think the current RISC-V thing is stonger than
> PowerPC, but maybe not as strong as say ARM64, but RISC-V memory
> ordering is still somewhat hazy to me.
> 
> Specifically, the sequence:
> 
> 	/* critical section s */
> 	WRITE_ONCE(x, 1);
> 	FENCE RW, W
> 	WRITE_ONCE(s.lock, 0);		/* store S */
> 	AMOSWAP %0, 1, r.lock		/* store R */
> 	FENCE R, RW
> 	WRITE_ONCE(y, 1);
> 	/* critical section r */
> 
> fully separates section s from section r, as in RW->RW ordering
> (possibly not as strong as smp_mb() though), while on PowerPC it would
> only impose TSO ordering between sections.
> 
> The AMOSWAP is a RmW and as such matches the W from the RW->W fence,
> similarly it marches the R from the R->RW fence, yielding an:
> 
> 	RW->  W
> 	    RmW
> 	    R  ->RW
> 
> ordering. It's the stores S and R that can be re-ordered, but not the
> sections themselves (same on PowerPC and many others).
> 
> Clarification from a RISC-V enabled person would be appreciated.
> 
> > then I think it's entirely reasonable to
> > 
> >         spin_unlock(&r);
> >         spin_lock(&s);
> > 
> > cannot be reordered.
> 
> I'm obviously completely in favour of that :-)

I don't think we should require the accesses to the actual lockwords to
be ordered here, as it becomes pretty onerous for relaxed LL/SC
architectures where you'd end up with an extra barrier either after the
unlock() or before the lock() operation. However, I remain absolutely in
favour of strengthening the ordering of the _critical sections_ guarded by
the locks to be RCsc.

Last time this came up, I think the RISC-V folks were generally happy to
implement whatever was necessary for Linux [1]. The thing that was stopping
us was Power (see CONFIG_ARCH_WEAK_RELEASE_ACQUIRE), wasn't it? I think
Michael saw quite a bit of variety in the impact on benchmarks [2] across
different machines. So the question is whether newer Power machines are less
affected to the degree that we could consider making this change again.

Will

[1] https://lore.kernel.org/lkml/11b27d32-4a8a-3f84-0f25-723095ef1076@nvidia.com/
[2] https://lore.kernel.org/lkml/87tvp3xonl.fsf@concordia.ellerman.id.au/