Summary:
Of course, compare_exchange_strong didn't actually do what I wanted.
Using a mutex is simpler and actually has the semantics we want: atomically get the current value, compare, and bail if the value is the same, or swap and continue.
Changelog: [Internal]
Reviewed By: shergin
Differential Revision: D19754451
fbshipit-source-id: 6b0aef217b235959af683ec5e31b07a0dd7bb040