Messages in this thread | | | Date | Fri, 10 Sep 2021 11:33:25 +0200 | From | Peter Zijlstra <> | Subject | Re: [tip:locking/core] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire |
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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().
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.
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