Auto merge of rust-lang#2194 - RalfJung:race, r=RalfJung

add interesting data race test

This interesting testcase came up in rust-lang/miri#2192.

Author: bors

tests/fail/data_race/fence_after_load.rs

@@ -0,0 +1,24 @@
+// compile-flags: -Zmiri-disable-isolation
+// ignore-windows: Concurrency on Windows is not supported yet.
+use std::sync::Arc;
+use std::sync::atomic::{AtomicUsize, Ordering, fence};
+use std::time::Duration;
+use std::thread;
+
+fn main() {
+    static mut V: u32 = 0;
+    let a = Arc::new(AtomicUsize::default());
+    let b = a.clone();
+    thread::spawn(move || {
+        unsafe { V = 1 }
+        b.store(1, Ordering::SeqCst);
+    });
+    thread::sleep(Duration::from_millis(100));
+    fence(Ordering::SeqCst);
+    // Imagine the other thread's actions happening here.
+    assert_eq!(a.load(Ordering::Relaxed), 1);
+    // The fence is useless, since it did not happen-after the `store` in the other thread.
+    // Hence this is a data race.
+    // Also see https://github.com/rust-lang/miri/issues/2192.
+    unsafe { V = 2 } //~ERROR Data race detected
+}

tests/fail/data_race/fence_after_load.stderr

@@ -0,0 +1,17 @@
+warning: thread support is experimental and incomplete: weak memory effects are not emulated.
+
+error: Undefined Behavior: Data race detected between Write on Thread(id = 0, name = "main") and Write on Thread(id = 1) at ALLOC (current vector clock = VClock, conflicting timestamp = VClock)
+  --> $DIR/fence_after_load.rs:LL:CC
+   |
+LL |     unsafe { V = 2 }
+   |              ^^^^^ Data race detected between Write on Thread(id = 0, name = "main") and Write on Thread(id = 1) at ALLOC (current vector clock = VClock, conflicting timestamp = VClock)
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+           
+   = note: inside `main` at $DIR/fence_after_load.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to previous error; 1 warning emitted
+