Attempt to release the lock and wake when we can't

Author: Azoy

stdlib/cmake/modules/AddSwiftStdlib.cmake

@@ -1830,34 +1830,34 @@ endfunction()
 #   Sources that are built when this library is being built for iOS
 #
 # SWIFT_SOURCES_DEPENDS_TVOS
-#   Sources that are built when this library is being built for macOS
+#   Sources that are built when this library is being built for tvOS
 #
 # SWIFT_SOURCES_DEPENDS_WATCHOS
-#   Sources that are built when this library is being built for macOS
+#   Sources that are built when this library is being built for watchOS
 #
 # SWIFT_SOURCES_DEPENDS_FREESTANDING
-#   Sources that are built when this library is being built for macOS
+#   Sources that are built when this library is being built for freestanding
 #
 # SWIFT_SOURCES_DEPENDS_FREEBSD
-#   Sources that are built when this library is being built for macOS
+#   Sources that are built when this library is being built for FreeBSD
 #
 # SWIFT_SOURCES_DEPENDS_OPENBSD
-#   Sources that are built when this library is being built for macOS
+#   Sources that are built when this library is being built for OpenBSD
 #
 # SWIFT_SOURCES_DEPENDS_LINUX
-#   Sources that are built when this library is being built for macOS
+#   Sources that are built when this library is being built for Linux
 #
 # SWIFT_SOURCES_DEPENDS_CYGWIN
-#   Sources that are built when this library is being built for macOS
+#   Sources that are built when this library is being built for Cygwin
 #
 # SWIFT_SOURCES_DEPENDS_HAIKU
-#   Sources that are built when this library is being built for macOS
+#   Sources that are built when this library is being built for Haiku
 #
 # SWIFT_SOURCES_DEPENDS_WASI
-#   Sources that are built when this library is being built for macOS
+#   Sources that are built when this library is being built for WASI
 #
 # SWIFT_SOURCES_DEPENDS_WINDOWS
-#   Sources that are built when this library is being built for macOS
+#   Sources that are built when this library is being built for Windows
 #
 # source1 ...
 #   Sources to add into this library.

stdlib/public/Synchronization/Mutex/LinuxImpl.swift

@@ -108,11 +108,18 @@ internal struct _MutexHandle: ~Copyable {
   borrowing func unlock() {
     // TODO: Is it worth caching this value in TLS?
     let selfId = _gettid()
-    // Unconditionally update the storage to be unlocked. At this point, the
-    // lock is acquirable.
-    let oldValue = storage.exchange(0, ordering: .releasing)
 
-    if _fastPath(oldValue == selfId) {
+    // Attempt to release the lock. We can only atomically release the lock in
+    // user-space when there are no other waiters. If there are waiters, the
+    // waiter bit is set and we need to inform the kernel that we're unlocking.
+    let (exchanged, oldValue) = storage.compareExchange(
+      expected: selfId,
+      desired: 0,
+      successOrdering: .releasing,
+      failureOrdering: .relaxed
+    )
+
+    if _fastPath(exchanged) {
       // No waiters, unlocked!
       return
     }