Add elision to pthread_mutex_{try,timed,un}lock

Add elision paths to the basic mutex locks.

The normal path has a check for RTM and upgrades the lock
to RTM when available. Trylocks cannot automatically upgrade,
so they check for elision every time.

We use a 4 byte value in the mutex to store the lock
elision adaptation state. This is separate from the adaptive
spin state and uses a separate field.

Condition variables currently do not support elision.

Recursive mutexes and condition variables may be supported at some point,
but are not in the current implementation. Also "trylock" will
not automatically enable elision unless some other lock call
has been already called on the lock.

This version does not use IFUNC, so it means every lock has one
additional check for elision. Benchmarking showed the overhead
to be negligible.

1 files changed, 17 insertions, 4 deletions

diff --git a/nptl/pthread_mutex_unlock.c b/nptl/pthread_mutex_unlock.c
index c0249f76ea..6914503626 100644
--- a/nptl/pthread_mutex_unlock.c
+++ b/nptl/pthread_mutex_unlock.c
@@ -23,6 +23,10 @@
 #include <lowlevellock.h>
 #include <stap-probe.h>
 
+#ifndef lll_unlock_elision
+#define lll_unlock_elision(a,b) ({ lll_unlock (a,b); 0; })
+#endif
+
 static int
 internal_function
 __pthread_mutex_unlock_full (pthread_mutex_t *mutex, int decr)
@@ -34,8 +38,9 @@ __pthread_mutex_unlock_usercnt (mutex, decr)
      pthread_mutex_t *mutex;
      int decr;
 {
-  int type = PTHREAD_MUTEX_TYPE (mutex);
-  if (__builtin_expect (type & ~PTHREAD_MUTEX_KIND_MASK_NP, 0))
+  int type = PTHREAD_MUTEX_TYPE_ELISION (mutex);
+  if (__builtin_expect (type &
+		~(PTHREAD_MUTEX_KIND_MASK_NP|PTHREAD_MUTEX_ELISION_FLAGS_NP), 0))
     return __pthread_mutex_unlock_full (mutex, decr);
 
   if (__builtin_expect (type, PTHREAD_MUTEX_TIMED_NP)
@@ -55,7 +60,14 @@ __pthread_mutex_unlock_usercnt (mutex, decr)
 
       return 0;
     }
-  else if (__builtin_expect (type == PTHREAD_MUTEX_RECURSIVE_NP, 1))
+  else if (__builtin_expect (type == PTHREAD_MUTEX_TIMED_ELISION_NP, 1))
+    {
+      /* Don't reset the owner/users fields for elision.  */
+      return lll_unlock_elision (mutex->__data.__lock,
+				      PTHREAD_MUTEX_PSHARED (mutex));
+    }
+  else if (__builtin_expect (PTHREAD_MUTEX_TYPE (mutex)
+			      == PTHREAD_MUTEX_RECURSIVE_NP, 1))
     {
       /* Recursive mutex.  */
       if (mutex->__data.__owner != THREAD_GETMEM (THREAD_SELF, tid))
@@ -66,7 +78,8 @@ __pthread_mutex_unlock_usercnt (mutex, decr)
 	return 0;
       goto normal;
     }
-  else if (__builtin_expect (type == PTHREAD_MUTEX_ADAPTIVE_NP, 1))
+  else if (__builtin_expect (PTHREAD_MUTEX_TYPE (mutex)
+			      == PTHREAD_MUTEX_ADAPTIVE_NP, 1))
     goto normal;
   else
     {