[compiler-rt][builtins] Add opt-in pthread_mutex_t locks to libatomic (…

…#95326)

When an uninstrumented libatomic is used with a TSan instrumented
memcpy, TSan may report a data race in circumstances where writes are
arguably safe.

This occurs because __atomic_compare_exchange won't be instrumented in
an uninstrumented libatomic, so TSan doesn't know that the subsequent
memcpy is race-free.

On the other hand, pthread_mutex_(un)lock will be intercepted by TSan,
meaning an uninstrumented libatomic will not report this false-positive.

pthread_mutexes also may try a number of different strategies to acquire
the lock, which may bound the amount of time a thread has to wait for a
lock during contention.

While pthread_mutex_lock has a larger overhead (due to the function call
and some dispatching), a dispatch to libatomic already predicates a lack
of performance guarantees.

compiler-rt/lib/builtins/CMakeLists.txt

@@ -237,6 +237,14 @@ if(NOT FUCHSIA AND NOT COMPILER_RT_BAREMETAL_BUILD)
   )
 endif()
 
+option(COMPILER_RT_LIBATOMIC_USE_PTHREAD
+  "Whether libatomic should use pthreads if available."
+  Off)
+
+if(COMPILER_RT_LIBATOMIC_USE_PTHREAD)
+  add_compile_definitions(_LIBATOMIC_USE_PTHREAD)
+endif()
+
 if(COMPILER_RT_HAS_ATOMIC_KEYWORD AND NOT COMPILER_RT_EXCLUDE_ATOMIC_BUILTIN)
   set(GENERIC_SOURCES
     ${GENERIC_SOURCES}

compiler-rt/lib/builtins/atomic.c

@@ -12,7 +12,7 @@
 //
 //  1) This code must work with C programs that do not link to anything
 //     (including pthreads) and so it should not depend on any pthread
-//     functions.
+//     functions. If the user wishes to opt into using pthreads, they may do so.
 //  2) Atomic operations, rather than explicit mutexes, are most commonly used
 //     on code where contended operations are rate.
 //
@@ -56,7 +56,17 @@ static const long SPINLOCK_MASK = SPINLOCK_COUNT - 1;
 // defined.  Each platform should define the Lock type, and corresponding
 // lock() and unlock() functions.
 ////////////////////////////////////////////////////////////////////////////////
-#if defined(__FreeBSD__) || defined(__DragonFly__)
+#if defined(_LIBATOMIC_USE_PTHREAD)
+#include <pthread.h>
+typedef pthread_mutex_t Lock;
+/// Unlock a lock.  This is a release operation.
+__inline static void unlock(Lock *l) { pthread_mutex_unlock(l); }
+/// Locks a lock.
+__inline static void lock(Lock *l) { pthread_mutex_lock(l); }
+/// locks for atomic operations
+static Lock locks[SPINLOCK_COUNT];
+
+#elif defined(__FreeBSD__) || defined(__DragonFly__)
 #include <errno.h>
 // clang-format off
 #include <sys/types.h>