FROMLIST: selftests/futex: Order calls in futex_requeue

Fixes a race between parent and child threads in futex_requeue.

Similar to fbf4dec7 ("selftests/futex: Order calls to
futex_lock_pi"), which fixed a flake in futex_lock_pi due to racing
between the parent and child threads.

The same issue can occur in the futex_requeue test, because it expects
waiterfn to make progress to futex_wait before the parent starts to
requeue. This is mitigated by the parent sleeping for WAKE_WAIT_US, but
it still fails occasionally. This can be reproduced by adding a sleep in
the waiterfn before futex_wait:

TAP version 13
1..2
not ok 1 futex_requeue simple returned: 0
not ok 2 futex_requeue simple returned: 0
not ok 3 futex_requeue many returned: 0
not ok 4 futex_requeue many returned: 0

Instead, replace the sleep with barriers to make the sequencing
explicit.

Fixes: 7cb5dd8e ("selftests: futex: Add futex compare requeue test")
Signed-off-by: Edward Liaw <edliaw@google.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: André Almeida <andrealmeid@igalia.com>

Link: https://lore.kernel.org/linux-kselftest/20240918231102.234253-1-edliaw@google.com/


Bug: 367134942
Bug: 260003172
Test: ./run_test_only.sh -t 'selftests kselftest_futex_requeue'
Change-Id: I8650c173bfd6333e00d0dede381a10310f2f42d1
Signed-off-by: Edward Liaw <edliaw@google.com>