s390/processor: Implement cpu_relax() with cpu serialization

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There are many loops in the form of

        while (READ_ONCE(*somelocation))
                cpu_relax();

Strictly speaking the architecture requires serialization instead of only a
compiler barrier in the loop so the READ_ONCE() will see an updated value.

However real hardware does not require this (see IBM z Systems Processor
Optimization Primer - FAQ [1]), but it is still recommended to add
serialization. Given that cpu_relax() is doing nothing useful, it does
not hurt to add the single and fast instruction which makes sure that
serialization happens, and such loops may be left a bit faster.

[1] https://community.ibm.com/community/user/viewdocument/microprocessor-optimization-primer

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

...

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Heiko Carstens 2026-05-08 17:02:48 +02:00 • committed by Alexander Gordeev

parent 24b3afcff4

commit 44e5edace5

1 changed files with 3 additions and 1 deletions

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arch/s390/include/asm/vdso/processor.h

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