Failure in stdlib/tst-arc4random-thread testing glibc 2.41 for release.

[Florian Weimer]

* Carlos O'Donell:

> Adhemerval,
>
> In testing glibc 2.41 for release I had this fail in a *very* surprising way:
> https://sourceware.org/glibc/wiki/Testing/Tests/stdlib/tst-arc4random-thread
>
> System is a local Lenovo P16v laptop under testing load:
>
> info: outer_threads=2 inner_threads=4
> info: arc4random: minimum of 163840 blob results expected
> info: arc4random: 163894 blob results observed
> error: arc4random: duplicate blob: "\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000" (16 bytes)
>   first source: thread -1, index 0
>   second source: thread -1, index 1
> info: arc4random_buf: minimum of 163840 blob results expected
> info: arc4random_buf: 163840 blob results observed
> info: arc4random_uniform: minimum of 163840 blob results expected
> info: arc4random_uniform: 163840 blob results observed
> error: 1 test failures

I can reproduce it on kernel 6.12.8 after a while running the test in a
loop, and multiple such loops in parallel (probably just to generate
load).  Thread -1 means that it happens after fork.  I see duplicates
not just with zeros.

First step would be to check if all the threads after fork receive
distinct per-thread states.  If not, there's probably a glitch in our
state allocation code.  Or maybe the state recovery from reclaim_stacks
produces duplicates.

Thanks,
Florian