commit 1fad074edd7010a5b383d62e05e198a77d967df4
author Ben Hamilton <benhamilton@google.com> Thu May 16 10:11:56 2024 -0600
committer Copybara-Service <copybara-worker@google.com> Thu May 16 15:17:42 2024 -0700
tree bdf7a6a091657b583fbe6e422aa0c98ce73081b7
parent 8f7e247ad7160db8260e3f08e2c996d892f2acd4

[ScopedSpinGuard] Use std::atomic::compare_exchange_strong() for spinlock

Previously, ScopedSpinGuard used std::atomic::compare_exchange_weak()
in a loop to implement a spinlock. After looping for the specified
number of nanoseconds, it would give up and return an error.

A few bugs have come in on ARM platforms (https://crbug.com/340980960,
http://b/296082201) which indicate that this can fail even in
single-threaded cases where nothing else has the spinlock.

From https://cbloomrants.blogspot.com/2011/07/07-14-11-compareexchangestrong-vs.html :

> compare_exchange_weak exists for LL-SC (load linked/store
> conditional) type architectures (Power, ARM, basically everything
> except x86), because on them compare_exchange_strong must be
> implemented as a loop, while compare_exchange_weak can be
> non-looping.

and:

https://en.cppreference.com/w/cpp/atomic/atomic/compare_exchange#Notes

> compare_exchange_weak is allowed to fail spuriously, that is, acts
> as if *this != expected even if they are equal. When a
> compare-and-exchange is in a loop, compare_exchange_weak will yield
> better performance on some platforms.
>
> When compare_exchange_weak would require a loop and
> compare_exchange_strong would not, compare_exchange_strong is
> preferable [...]

My conclusion is that this logic needs to use
`compare_exchange_strong` to avoid spurious failures on ARM in the
common case when there's no other thread holding the spinlock.

Change-Id: I2a08031db6b219d7d14a5cd02b3634985f81ab06

Bug: b:340980960
Change-Id: I8686cbf0c9cfa7b61df90976b2121ca8558ce372
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/5545257
Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Ben Hamilton <benhamilton@google.com>
GitOrigin-RevId: d588c50b16f735b0c069538651d06bdd9490c6f3