Messages in this thread | | | Subject | Re: [RFC PATCH v5 5/5] riscv/cmpxchg: Implement xchg for variables of size 1 and 2 | From | Leonardo Brás <> | Date | Wed, 30 Aug 2023 18:59:46 -0300 |
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On Thu, 2023-08-10 at 09:23 -0700, Palmer Dabbelt wrote: > On Thu, 10 Aug 2023 09:04:04 PDT (-0700), leobras@redhat.com wrote: > > On Thu, 2023-08-10 at 08:51 +0200, Arnd Bergmann wrote: > > > On Thu, Aug 10, 2023, at 06:03, Leonardo Bras wrote: > > > > xchg for variables of size 1-byte and 2-bytes is not yet available for > > > > riscv, even though its present in other architectures such as arm64 and > > > > x86. This could lead to not being able to implement some locking mechanisms > > > > or requiring some rework to make it work properly. > > > > > > > > Implement 1-byte and 2-bytes xchg in order to achieve parity with other > > > > architectures. > > > > > > > > Signed-off-by: Leonardo Bras <leobras@redhat.com> > > > > > > > Hello Arnd Bergmann, thanks for reviewing! > > > > > Parity with other architectures by itself is not a reason to do this, > > > in particular the other architectures you listed have the instructions > > > in hardware while riscv does not. > > > > Sure, I understand RISC-V don't have native support for xchg on variables of > > size < 4B. My argument is that it's nice to have even an emulated version for > > this in case any future mechanism wants to use it. > > > > Not having it may mean we won't be able to enable given mechanism in RISC-V. > > IIUC the ask is to have a user within the kernel for these functions. > That's the general thing to do, and last time this came up there was no > in-kernel use of it -- the qspinlock stuff would, but we haven't enabled > it yet because we're worried about the performance/fairness stuff that > other ports have seen and nobody's got concrete benchmarks yet (though > there's another patch set out that I haven't had time to look through, > so that may have changed). > > So if something uses these I'm happy to go look closer.
IIUC patches 4 & 5 will be used by qspinlock, which may not be done yet, so we don't have an use for them for the time being.
Otherwise, any comments on patches 1, 2 & 3?
> > > > Emulating the small xchg() through cmpxchg() is particularly tricky > > > since it's easy to run into a case where this does not guarantee > > > forward progress. > > > > > > > Didn't get this part: > > By "emulating small xchg() through cmpxchg()", did you mean like emulating an > > xchg (usually 1 instruction) with lr & sc (same used in cmpxchg) ? > > > > If so, yeah, it's a fair point: in some extreme case we could have multiple > > threads accessing given cacheline and have sc always failing. On the other hand, > > there are 2 arguments on that: > > > > 1 - Other architectures, (such as powerpc, arm and arm64 without LSE atomics) > > also seem to rely in this mechanism for every xchg size. Another archs like csky > > and loongarch use asm that look like mine to handle size < 4B xchg. > > > > > > > This is also something that almost no architecture > > > specific code relies on (generic qspinlock being a notable exception). > > > > > > > 2 - As you mentioned, there should be very little code that will actually make > > use of xchg for vars < 4B, so it should be safe to assume its fine to not > > guarantee forward progress for those rare usages (like some of above mentioned > > archs). > > > > > I would recommend just dropping this patch from the series, at least > > > until there is a need for it. > > > > While I agree this is a valid point, I believe its more interesting to have it > > implemented if any future mechanism wants to make use of this. > > > > > > Thanks! > > Leo >
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