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| -rw-r--r-- | library/std/src/thread/parker/futex.rs | 20 |
1 files changed, 20 insertions, 0 deletions
diff --git a/library/std/src/thread/parker/futex.rs b/library/std/src/thread/parker/futex.rs index 2d7a7770508..a5d4927dcc5 100644 --- a/library/std/src/thread/parker/futex.rs +++ b/library/std/src/thread/parker/futex.rs @@ -11,6 +11,26 @@ pub struct Parker { state: AtomicI32, } +// Notes about memory ordering: +// +// Memory ordering is only relevant for the relative ordering of operations +// between different variables. Even Ordering::Relaxed guarantees a +// monotonic/consistent order when looking at just a single atomic variable. +// +// So, since this parker is just a single atomic variable, we only need to look +// at the ordering guarantees we need to provide to the 'outside world'. +// +// The only memory ordering guarantee that parking and unparking provide, is +// that things which happened before unpark() are visible on the thread +// returning from park() afterwards. Otherwise, it was effectively unparked +// before unpark() was called while still consuming the 'token'. +// +// In other words, unpark() needs to synchronize with the part of park() that +// consumes the token and returns. +// +// This is done with a release-acquire synchronization, by using +// Ordering::Release when writing NOTIFIED (the 'token') in unpark(), and using +// Ordering::Acquire when checking for this state in park(). impl Parker { #[inline] pub const fn new() -> Self { |