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? 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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