From cb57484dcaeb4881c73f8a1174d4d5661ca2bbe2 Mon Sep 17 00:00:00 2001
From: Alex Crichton <alex@alexcrichton.com>
Date: Wed, 3 Apr 2019 12:18:38 -0700
Subject: std: Avoid usage of `Once` in `Instant`

This commit removes usage of `Once` from the internal implementation of
time utilities on OSX and Windows. It turns out that we accidentally hit
a deadlock today (#59020) via events that look like:

* A thread invokes `park_timeout`
* Internally, only on OSX, `park_timeout` calls `Instant::elapsed`
* Inside of `Instant::elapsed` on OSX we enter a `Once` to initialize
  global timer data
* Inside of `Once`, it attempts to `park`

This means on the same stack frame, when there's contention, we're
calling `park` from inside `park_timeout`, causing a deadlock!

The solution implemented in this commit was to remove usage of `Once`
and instead just do a small dance with atomics. There's no real need we
need to guarantee that the global information is only learned once, only
that it's only *stored* once. This implementation may have multiple
threads invoke `mach_timebase_info`, but only one will store the global
information which will amortize the cost for all other threads.

A similar fix has been applied to windows to be uniform across our
implementations, but looking at the code on Windows no deadlock was
possible. This is purely just a consistency update for Windows and in
theory a slightly leaner implementation.

Closes #59020
---
 src/test/run-pass/issue-59020.rs | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)
 create mode 100644 src/test/run-pass/issue-59020.rs

(limited to 'src/test')

diff --git a/src/test/run-pass/issue-59020.rs b/src/test/run-pass/issue-59020.rs
new file mode 100644
index 00000000000..a2b11764a2f
--- /dev/null
+++ b/src/test/run-pass/issue-59020.rs
@@ -0,0 +1,27 @@
+// edition:2018
+// run-pass
+// ignore-emscripten no threads support
+
+use std::thread;
+use std::time::Duration;
+
+fn main() {
+    let t1 = thread::spawn(|| {
+        let sleep = Duration::new(0,100_000);
+        for _ in 0..100 {
+            println!("Parking1");
+            thread::park_timeout(sleep);
+        }
+    });
+
+    let t2 = thread::spawn(|| {
+        let sleep = Duration::new(0,100_000);
+        for _ in 0..100 {
+            println!("Parking2");
+            thread::park_timeout(sleep);
+        }
+    });
+
+    t1.join().expect("Couldn't join thread 1");
+    t2.join().expect("Couldn't join thread 2");
+}
-- 
cgit 1.4.1-3-g733a5