From 3712bb68c4f76161b54dcade7c1497b3ffc32e11 Mon Sep 17 00:00:00 2001
From: Paul Dicker <pitdicker@gmail.com>
Date: Fri, 25 Oct 2019 10:01:27 +0200
Subject: Mention park guarantee

---
 src/libstd/sync/once.rs | 4 ++++
 1 file changed, 4 insertions(+)

(limited to 'src/libstd/sync')

diff --git a/src/libstd/sync/once.rs b/src/libstd/sync/once.rs
index c135471e2f2..bdb941cff52 100644
--- a/src/libstd/sync/once.rs
+++ b/src/libstd/sync/once.rs
@@ -469,6 +469,10 @@ fn wait(state_and_queue: &AtomicUsize, current_state: usize) {
     // dangling reference). Guard against spurious wakeups by reparking
     // ourselves until we are signaled.
     while !node.signaled.load(Ordering::Acquire) {
+        // If the managing thread happens to signal and unpark us before we can
+        // park ourselves, the result could be this thread never gets unparked.
+        // Luckily `park` comes with the guarantee that if it got an `unpark`
+        // just before on an unparked thread is does not park.
         thread::park();
     }
 }
-- 
cgit 1.4.1-3-g733a5