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

[Boqun Feng]

On Fri, Sep 10, 2021 at 11:33:25AM +0200, Peter Zijlstra wrote:
> On Fri, Sep 10, 2021 at 08:01:14AM +0800, Boqun Feng wrote:
> > On Thu, Sep 09, 2021 at 01:03:18PM -0400, Dan Lustig wrote:
> > > On 9/9/2021 9:35 AM, Will Deacon wrote:
> > > > On Thu, Sep 09, 2021 at 09:25:30AM +0200, Peter Zijlstra wrote:
> 
> > > >> 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).
> 
> > > I agree with Will here.  If the AMOSWAP above is actually implemented with
> > > a RISC-V AMO, then the two critical sections will be separated as if RW,RW,
> > > as Peter described.  If instead it's implemented using LR/SC, then RISC-V
> > 
> > Just out of curiosity, in the following code, can the store S and load L
> > be reordered?
> > 
> > 	WRITE_ONCE(x, 1); // store S
> > 	FENCE RW, W
> >  	WRITE_ONCE(s.lock, 0); // unlock(s)
> >  	AMOSWAP %0, 1, s.lock  // lock(s)
> > 	FENCE R, RW
> > 	r1 = READ_ONCE(y); // load L
> > 
> > I think they can, because neither "FENCE RW, W" nor "FENCE R, RW" order
> > them.
> 
> I'm confused by your argument, per the above quoted section, those
> fences and the AMO combine into a RW,RW ordering which is (as per the
> later clarification) multi-copy-atomic, aka smp_mb().
>

Right, my question is more about the reasoning about why fence rw,w +
AMO + fence r,rw act as a fence rw,rw. Another related question, can
fence rw,w + store + fence w,rw act as a fence rw,rw by the similar
reasoning? IOW, will the two loads in the following be reordered?

	r1 = READ_ONCE(x);
	FENCE RW, W
	WRITE_ONCE(z, 1);
	FENCE W, RW
	r2 = READ_ONCE(y);

again, this is more like a question out of curiosity, not that I find
this pattern is useful.

Regards,
Boqun

> As such, S and L are not allowed to be re-ordered in the given scenario.
> 
> > Note that the reordering is allowed in LKMM, because unlock-lock
> > only need to be as strong as RCtso.
> 
> Risc-V is strictly stronger than required in this instance. Given the
> current lock implementation. Daniel pointed out that if the atomic op
> were LL/SC based instead of AMO it would end up being RCtso.
>