[libc-commits] [libc] [libc] rework mutex (PR #92168)

[Nick Desaulniers via libc-commits]

================
@@ -9,114 +9,72 @@
 #ifndef LLVM_LIBC_SRC___SUPPORT_THREADS_LINUX_MUTEX_H
 #define LLVM_LIBC_SRC___SUPPORT_THREADS_LINUX_MUTEX_H
 
+#include "hdr/types/pid_t.h"
+#include "src/__support/CPP/optional.h"
 #include "src/__support/threads/linux/futex_utils.h"
+#include "src/__support/threads/linux/raw_mutex.h"
 #include "src/__support/threads/mutex_common.h"
 
 namespace LIBC_NAMESPACE {
-struct Mutex {
+
+// TODO: support shared/recursive/robust mutexes.
+class Mutex final : private internal::RawMutex {
+  // reserved timed, may be useful when combined with other flags.
   unsigned char timed;
   unsigned char recursive;
   unsigned char robust;
+  unsigned char pshared;
 
-  void *owner;
+  // TLS address may not work across forked processes. Use thread id instead.
+  pid_t owner;
   unsigned long long lock_count;
 
-  Futex futex_word;
-
-  enum class LockState : FutexWordType {
-    Free,
-    Locked,
-    Waiting,
-  };
-
 public:
-  constexpr Mutex(bool istimed, bool isrecursive, bool isrobust)
-      : timed(istimed), recursive(isrecursive), robust(isrobust),
-        owner(nullptr), lock_count(0),
-        futex_word(FutexWordType(LockState::Free)) {}
-
-  static MutexError init(Mutex *mutex, bool istimed, bool isrecur,
-                         bool isrobust) {
-    mutex->timed = istimed;
+  LIBC_INLINE constexpr Mutex(bool is_timed, bool is_recursive, bool is_robust,
+                              bool is_pshared)
+      : internal::RawMutex(), timed(is_timed), recursive(is_recursive),
+        robust(is_robust), pshared(is_pshared), owner(0), lock_count(0) {}
+
+  LIBC_INLINE static MutexError init(Mutex *mutex, bool is_timed, bool isrecur,
+                                     bool isrobust, bool is_pshared) {
+    internal::RawMutex::init(mutex);
+    mutex->timed = is_timed;
     mutex->recursive = isrecur;
     mutex->robust = isrobust;
-    mutex->owner = nullptr;
+    mutex->pshared = is_pshared;
+    mutex->owner = 0;
     mutex->lock_count = 0;
-    mutex->futex_word.set(FutexWordType(LockState::Free));
     return MutexError::NONE;
   }
 
-  static MutexError destroy(Mutex *) { return MutexError::NONE; }
-
-  MutexError reset();
+  LIBC_INLINE static MutexError destroy(Mutex *) { return MutexError::NONE; }
----------------
nickdesaulniers wrote:

Should we be setting the `lock_count` and `owner` to `0`?

Should `RawMutex` have a destroy method as well? Do we need to do anything to clean up the `RawMutex`'s Futex?

https://github.com/llvm/llvm-project/pull/92168