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

[Peter Zijlstra]

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 :-)