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Ben Hamilton d588c50b16 [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: I2a08031db6b219d7d14a5cd02b3634985f81ab06
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>

2024-05-16 22:16:59 +00:00

build

Roll mini_chromium to pick up the latest version of base::span.

2024-04-11 16:00:45 +00:00

client

ios: Use correct address when storing register memory.

2024-05-09 18:56:10 +00:00

compat

Convert Crashpad to use ARC

2023-06-30 15:55:24 +00:00

doc

doc: Upgrade the crashpad-home App Engine app to the go121 flex runtime