Re: [RFC PATCH v5 5/5] riscv/cmpxchg: Implement xchg for variables of size 1 and 2

[Palmer Dabbelt]

On Thu, 10 Aug 2023 09:04:04 PDT (-0700), leobras@xxxxxxxxxx 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@xxxxxxxxxx>
>
> 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.

> > 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