Re: [PATCH] locking/atomic: Make test_and_*_bit() ordered on failure

[Jon Nettleton]

On Tue, Aug 16, 2022 at 7:38 PM Will Deacon <will@xxxxxxxxxx> wrote:
>
> On Tue, Aug 16, 2022 at 11:30:45PM +0900, Hector Martin wrote:
> > On 16/08/2022 23.04, Will Deacon wrote:
> > >> diff --git a/include/asm-generic/bitops/atomic.h b/include/asm-generic/bitops/atomic.h
> > >> index 3096f086b5a3..71ab4ba9c25d 100644
> > >> --- a/include/asm-generic/bitops/atomic.h
> > >> +++ b/include/asm-generic/bitops/atomic.h
> > >> @@ -39,9 +39,6 @@ arch_test_and_set_bit(unsigned int nr, volatile unsigned long *p)
> > >>    unsigned long mask = BIT_MASK(nr);
> > >>
> > >>    p += BIT_WORD(nr);
> > >> -  if (READ_ONCE(*p) & mask)
> > >> -          return 1;
> > >> -
> > >>    old = arch_atomic_long_fetch_or(mask, (atomic_long_t *)p);
> > >>    return !!(old & mask);
> > >>  }
> > >> @@ -53,9 +50,6 @@ arch_test_and_clear_bit(unsigned int nr, volatile unsigned long *p)
> > >>    unsigned long mask = BIT_MASK(nr);
> > >>
> > >>    p += BIT_WORD(nr);
> > >> -  if (!(READ_ONCE(*p) & mask))
> > >> -          return 0;
> > >> -
> > >>    old = arch_atomic_long_fetch_andnot(mask, (atomic_long_t *)p);
> > >>    return !!(old & mask);
> > >
> > > I suppose one sad thing about this is that, on arm64, we could reasonably
> > > keep the READ_ONCE() path with a DMB LD (R->RW) barrier before the return
> > > but I don't think we can express that in the Linux memory model so we
> > > end up in RmW territory every time.
> >
> > You'd need a barrier *before* the READ_ONCE(), since what we're trying
> > to prevent is a consumer from writing to the value without being able to
> > observe the writes that happened prior, while this side read the old
> > value. A barrier after the READ_ONCE() doesn't do anything, as that read
> > is the last memory operation in this thread (of the problematic sequence).
>
> Right, having gone back to your litmus test, I now realise it's the "SB"
> shape from the memory ordering terminology. It's funny because the arm64
> acquire/release instructions are RCsc and so upgrading the READ_ONCE()
> to an *arm64* acquire instruction would work for your specific case, but
> only because the preceeding store is a release.
>
> > At that point, I'm not sure DMB LD / early read / LSE atomic would be
> > any faster than just always doing the LSE atomic?
>
> It depends a lot on the configuration of the system and the state of the
> relevant cacheline, but generally avoiding an RmW by introducing a barrier
> is likely to be a win. It just gets ugly here as we'd want to avoid the
> DMB in the case where we end up doing the RmW. Possibly we could do
> something funky like a test-and-test-and-test-and-set (!) where we do
> the DMB+READ_ONCE() only if the first READ_ONCE() has the bit set, but
> even just typing that is horrible and I'd _absolutely_ want to see perf
> numbers to show that it's a benefit once you start taking into account
> things like branch prediction.
>
> Anywho, since Linus has applied the patch and it should work, this is
> just an interesting aside.
>
> Will
>

It is moot if Linus has already taken the patch, but with a stock
kernel config I am
still seeing a slight performance dip but only ~1-2% in the specific
tests I was running.
Sorry about the noise I will need to look at my kernel builder and see what went
wrong when I have more time.

Cheers,
Jon