Re: [tip:locking/core] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire
From: Boqun Feng
Date: Fri Sep 10 2021 - 06:06:03 EST
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
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.
> 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.