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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,b:296082201 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> (cherry picked from commit d588c50b16f735b0c069538651d06bdd9490c6f3) Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/5631682 Reviewed-by: Justin Cohen <justincohen@chromium.org>