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: > > > [+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. > > > > To first order, RISC-V's memory model is very similar to ARMv8's. It > > is "other-multi-copy-atomic", unlike Power, and respects dependencies. > > It also has AMOs and LR/SC with optional RCsc acquire or release > > semantics. There's no need to worry about RISC-V somehow pushing the > > boundaries of weak memory ordering in new ways. > > > > The tricky part is that unlike ARMv8, RISC-V doesn't have load-acquire > > or store-release opcodes at all. Only AMOs and LR/SC have acquire or > > release options. That means that while certain operations like swap > > can be implemented with native RCsc semantics, others like store-release > > have to fall back on fences and plain writes. > > > > That's where the complexity came up last time this was discussed, at > > least as it relates to RISC-V: how to make sure the combination of RCsc > > atomics and plain operations+fences gives the semantics everyone is > > asking for here. And to be clear there, I'm not asking for LKMM to > > weaken anything about critical section ordering just for RISC-V's sake. > > TSO/RCsc ordering between critical sections is a perfectly reasonable > > model in my opinion. I just want to make sure RISC-V gets it right > > given whatever the decision is. > > > > >>> 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. > > > > 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. Note that the reordering is allowed in LKMM, because unlock-lock > only need to be as strong as RCtso. > > Moreover, how about the outcome of the following case: > > { > r1, r2 are registers (variables) on each CPU, X, Y are memory > locations, and initialized as 0 > } > > CPU 0 > ===== > AMOSWAP r1, 1, X > FENCE R, RW > r2 = READ_ONCE(Y); > > CPU 1 > ===== > WRITE_ONCE(Y, 1); > FENCE RW, RW > r2 = READ_ONCE(X); > > can we observe the result where r2 on CPU0 is 0 while r2 on CPU1 is 1? > As reminded by Andrea, what I meant to ask here is: can we observer the result where r2 on CPU0 is 0 while r2 on CPU1 is 0? Regards, Boqun > Regards, > Boqun > > > gives only TSO (R->R, R->W, W->W), because the two pieces of the AMO are > > split, and that breaks the chain. Getting full RW->RW between the critical > > sections would therefore require an extra fence. Also, the accesses to the > > lockwords themselves would not be ordered without an extra fence. > > > > > 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. > > > > Yes, as I said above, RISC-V will implement what is needed to make this work. > > > > Dan > > > > > Will > > > > > > [1] https://lore.kernel.org/lkml/11b27d32-4a8a-3f84-0f25-723095ef1076@xxxxxxxxxx/ > > > [2] https://lore.kernel.org/lkml/87tvp3xonl.fsf@xxxxxxxxxxxxxxxxxxxxxxxx/
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