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

[Leonardo Brás]

On Thu, 2023-08-10 at 09:23 -0700, Palmer Dabbelt wrote:
> On Thu, 10 Aug 2023 09:04:04 PDT (-0700), leobras at 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 at 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
>