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

[Guo Ren]

On Fri, Aug 11, 2023 at 3:13 AM Arnd Bergmann <arnd at arndb.de> wrote:
>
> On Thu, Aug 10, 2023, at 18:23, 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.
> >>
> >>> 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).
>
> Right. In particular the qspinlock is a good example for something
> where having the emulated 16-bit xchg() may end up less efficient
> than a natively supported instruction.
The xchg() efficiency depends on micro-architecture. and the number of
instructions is not the key, even one instruction would be separated
into several micro-ops. I thought the Power guys won't agree with this
view :)

>
> The xchg() here is a performance optimization for CPUs that can
> do this without touching the other half of the 32-bit word.
It's useless on a non-SMT system because all operations are cacheline
based. (Ps: Because xchg() has a load semantic, CHI's "Dirty Partial"
& "Clean Empty" can't help anymore.)

>
> >>
> >> 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.
>
> I think you misread the arm64 code, which should use native instructions
> for all sizes, in both the armv8.0 and LSE atomics.
>
> PowerPC does use the masking for xchg, but I suspect there are no
> actual users, at least it actually has its own qspinlock implementation
> that avoids xchg().
PowerPC still needs similar things, see publish_tail_cpu(), and more
complex cmpxchg semantics.

Paravrit qspinlock and CNA qspinlock still need more:
 - xchg8 (RCsc)
 - cmpxchg8/16_relaxed
 - cmpxchg8/16_release (Rcpc)
 - cmpxchg8_acquire (RCpc)
 - cmpxchg8 (RCsc)

>
> >>>  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 don't this this is a safe assumption, we've had endless discussions
> about using qspinlock on architectures without a native xchg(), which
> needs either hardware guarantees or special countermeasures in xchg() itself
> to avoid this.
>
> What I'd actually like to do here is to remove the special 8-bit and
> 16-bit cases from the xchg() and cmpxchg() interfaces at all, leaving
It needs to modify qspinlock, paravirt_qspinlock, and CNA_qspinlock
code to prevent using 8-bit/16-bit xchg/cmpxchg, and cleanup all
architectures' cmpxchg.h. What you do is just get them out of the
common atomic.h, but architectures still need to solve them and
connect to the qspinlock series.

> only fixed 32-bit and native wordsize (either 32 or 64) as the option,
> while dealing with the others the same way we treat the fixed
> 64-bit cases that hardcode the 64-bit argument types and are only
> usable on architectures that provide them.
>
>     Arnd



-- 
Best Regards
 Guo Ren