[PATCH] Improve performance of IO locks

[Florian Weimer]

* Thomas Fitzsimmons:

> Hi Mark,
>
> Mark Wielaard <mark@klomp.org> writes:
>
>> On Mon, Aug 01, 2022 at 11:06:07AM +0000, Wilco Dijkstra via Libc-alpha wrote:
>>> Improve performance of recursive IO locks by adding a fast path for
>>> the single-threaded case. To reduce the number of memory accesses for
>>> locking/unlocking, only increment the recursion counter if the lock
>>> is already taken. 
>>> 
>>> On Neoverse V1, a microbenchmark with many small freads improved by
>>> 2.9 times. Multithreaded performance improved by 2%.
>>
>> Strangely this seems to have broken the glibc-debian-ppc64 buildbot:
>> https://builder.sourceware.org/buildbot/#/builders/glibc-debian-ppc64
>>
>> I don't see how this commit can cause a failure just on debian-ppc64
>> (all other distro/arches are fine).
>>
>> And the corresponding bunsen test results don't really show why.
>> https://builder.sourceware.org/testrun/498f51f327afdd7030516455b709a31a0038b432
>> Most of the .out files are empty, but some indicate an segmentation fault.
>>
>> Comparing to the build before only shows test result diffs, no
>> configuration differences.
>> https://builder.sourceware.org/testrun-diffs?commitish=498f51f327afdd7030516455b709a31a0038b432&commitish=58fd9d63b078b6bbfdba45135c4021038f33534e
>>
>> I don't have access to the buildbot, so cannot easily investigate more.
>>
>> Tom, could you have a look and see if you can find out more? Does just
>> reverting commit c51c483d2b8ae66fe31a12509aedae02a6982ced make things
>> OK again?
>
> Yes reverting that commit, the result is:
>
> Summary of test results:
>     317 PASS
>      10 UNSUPPORTED
>       2 XPASS
>
> Without the reversion:
>
> Summary of test results:
>     256 FAIL
>      68 PASS
>       3 UNSUPPORTED
>       2 XFAIL
>
> I looked at elf/unload as an example; it's segfaulting in
> _dl_relocate_object, backtrace attached; not sure what else to check.

I don't see this on powerpc64, with a toolchain based on GCC 8.2 and
binutils 2.30.  I'm at a loss how these things could be related.

Thanks,
Florian