testsuite: tolerate compare_exchange_weak spurious failure

[Alexandre Oliva]

On Mar 11, 2026, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:

> On Wed, 11 Mar 2026 at 10:56, Alexandre Oliva <oliva@adacore.com> wrote:
>> 
>> On Mar 11, 2026, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>> 
>> > On Wed, 11 Mar 2026, 05:29 Alexandre Oliva, <oliva@adacore.com> wrote:
>> >>
>> >> compare_exchange_weak is allowed to fail spuriously, and we're
>> >> actually hitting such spurious failures quite often on ppc-vx7r2,
>> >> presumably when random interrupts hit between ll and sc.
>> >>
>> >> Tolerate one such failure, hopefully that will suffice.
>> 
>> > This is a bit of a problem, because we can't tell if the padding was
>> > cleared or not. If it was not clear, the first compare_exchange_weak would
>> > clear it, and the refund would succeed. But we want the test to fail in
>> > that case.
>> 
>> Erhm...  My understanding is that, when it returns false, no memory is
>> changed, so the second attempt would start from exactly the same state
>> as the first.

> "Remarks: A weak compare-and-exchange operation may fail spuriously.
> That is, even when the contents of memory referred to by expected and
> ptr are equal, it may return false and store back to expected the same
> memory contents that were originally there."

> So `expected` is updated on a spurious failure, even if that doesn't
> change any bits.

Yeah, but if it stores back what was there to begin with, how can that
be a problem?  Is there any reason to expect that "store back [of] the
same memory contents" to not actually mean that, and end up messing with
padding bits?

Anyhow, your near-rewrite of the test is also passing consistently.  I
suspect it's not just because we're no longer likely to get the timing
just right for an interrupt to cause a spurious failure.  I needed a few
tens of retries to hit the problem, now I'm up to 500 without any hits.

-- 
Alexandre Oliva, happy hacker            https://blog.lx.oliva.nom.br/
Free Software Activist     FSFLA co-founder     GNU Toolchain Engineer
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