Make crossbeam-epoch compatible with ThreadSanitizer

ci/tsan

@@ -1,8 +1,5 @@
 # TSAN suppressions file for crossbeam
 
-# The epoch-based GC uses fences.
-race:crossbeam_epoch
-
 # Push and steal operations in crossbeam-deque may cause data races, but such
 # data races are safe. If a data race happens, the value read by `steal` is
 # forgotten and the steal operation is then retried.

crossbeam-epoch/build.rs

@@ -0,0 +1,13 @@
+// The rustc-cfg emitted by the build script are *not* public API.
+
+use std::env;
+
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+
+    // `cfg(sanitize = "..")` is not stabilized.
+    let sanitize = env::var("CARGO_CFG_SANITIZE").unwrap_or_default();
+    if sanitize.contains("thread") {
+        println!("cargo:rustc-cfg=crossbeam_sanitize_thread");
+    }
+}

crossbeam-epoch/src/internal.rs

@@ -248,10 +248,18 @@ impl Global {
                     if local_epoch.is_pinned() && local_epoch.unpinned() != global_epoch {
                         return global_epoch;
                     }
+
+                    // TODO: This is stronger than the actual one because the fence is only emitted
+                    // if the loop is not early exited.
+                    if cfg!(crossbeam_sanitize_thread) {
+                        local.epoch.load(Ordering::Acquire);
+                    }
                 }
             }
         }
-        atomic::fence(Ordering::Acquire);
+        if !cfg!(crossbeam_sanitize_thread) {
+            atomic::fence(Ordering::Acquire);
+        }
 
         // All pinned participants were pinned in the current global epoch.
         // Now let's advance the global epoch...