[PATCH v4 32/37] nptl: pthread_mutex_lock, pthread_mutex_unock single-threaded optimization

[Florian Weimer via Libc-alpha <libc-alpha@sourceware.org>]

This is optimization is similar in spirit to the SINGLE_THREAD_P check
in the malloc implementation.  Doing this in generic code allows us
to prioritize those cases which are likely to occur in single-threaded
programs (normal and recursive mutexes).

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
---
 nptl/pthread_mutex_cond_lock.c |  1 +
 nptl/pthread_mutex_lock.c      | 25 ++++++++++++++++++++++---
 nptl/pthread_mutex_unlock.c    | 17 ++++++++++++++++-
 3 files changed, 39 insertions(+), 4 deletions(-)

diff --git a/nptl/pthread_mutex_cond_lock.c b/nptl/pthread_mutex_cond_lock.c
index 2f0771302f..3386bd689b 100644
--- a/nptl/pthread_mutex_cond_lock.c
+++ b/nptl/pthread_mutex_cond_lock.c
@@ -2,6 +2,7 @@
 
 #define LLL_MUTEX_LOCK(mutex) \
   lll_cond_lock ((mutex)->__data.__lock, PTHREAD_MUTEX_PSHARED (mutex))
+#define LLL_MUTEX_LOCK_OPTIMIZED(mutex) LLL_MUTEX_LOCK (mutex)
 
 /* Not actually elided so far. Needed? */
 #define LLL_MUTEX_LOCK_ELISION(mutex)  \
diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c
index f0de7b7fd6..8649a92ffb 100644
--- a/nptl/pthread_mutex_lock.c
+++ b/nptl/pthread_mutex_lock.c
@@ -30,8 +30,27 @@
 /* Some of the following definitions differ when pthread_mutex_cond_lock.c
    includes this file.  */
 #ifndef LLL_MUTEX_LOCK
-# define LLL_MUTEX_LOCK(mutex) \
+/* lll_lock with single-thread optimization.  */
+static inline void
+lll_mutex_lock_optimized (pthread_mutex_t *mutex)
+{
+  /* The single-threaded optimization is only valid for private
+     mutexes.  For process-shared mutexes, the mutex could be in a
+     shared mapping, so synchronization with another process is needed
+     even without any threads.  If the lock is already marked as
+     acquired, POSIX requires that pthread_mutex_lock deadlocks for
+     normal mutexes, so skip the optimization in that case as
+     well.  */
+  int private = PTHREAD_MUTEX_PSHARED (mutex);
+  if (private == LLL_PRIVATE && SINGLE_THREAD_P && mutex->__data.__lock == 0)
+    mutex->__data.__lock = 1;
+  else
+    lll_lock (mutex->__data.__lock, private);
+}
+
+# define LLL_MUTEX_LOCK(mutex)						\
   lll_lock ((mutex)->__data.__lock, PTHREAD_MUTEX_PSHARED (mutex))
+# define LLL_MUTEX_LOCK_OPTIMIZED(mutex) lll_mutex_lock_optimized (mutex)
 # define LLL_MUTEX_TRYLOCK(mutex) \
   lll_trylock ((mutex)->__data.__lock)
 # define LLL_ROBUST_MUTEX_LOCK_MODIFIER 0
@@ -64,7 +83,7 @@ __pthread_mutex_lock (pthread_mutex_t *mutex)
       FORCE_ELISION (mutex, goto elision);
     simple:
       /* Normal mutex.  */
-      LLL_MUTEX_LOCK (mutex);
+      LLL_MUTEX_LOCK_OPTIMIZED (mutex);
       assert (mutex->__data.__owner == 0);
     }
 #if ENABLE_ELISION_SUPPORT
@@ -99,7 +118,7 @@ __pthread_mutex_lock (pthread_mutex_t *mutex)
 	}
 
       /* We have to get the mutex.  */
-      LLL_MUTEX_LOCK (mutex);
+      LLL_MUTEX_LOCK_OPTIMIZED (mutex);
 
       assert (mutex->__data.__owner == 0);
       mutex->__data.__count = 1;
diff --git a/nptl/pthread_mutex_unlock.c b/nptl/pthread_mutex_unlock.c
index 3b5ccdacf9..655093ee9a 100644
--- a/nptl/pthread_mutex_unlock.c
+++ b/nptl/pthread_mutex_unlock.c
@@ -28,6 +28,21 @@ static int
 __pthread_mutex_unlock_full (pthread_mutex_t *mutex, int decr)
      __attribute_noinline__;
 
+/* lll_lock with single-thread optimization.  */
+static inline void
+lll_mutex_unlock_optimized (pthread_mutex_t *mutex)
+{
+  /* The single-threaded optimization is only valid for private
+     mutexes.  For process-shared mutexes, the mutex could be in a
+     shared mapping, so synchronization with another process is needed
+     even without any threads.  */
+  int private = PTHREAD_MUTEX_PSHARED (mutex);
+  if (private == LLL_PRIVATE && SINGLE_THREAD_P)
+    mutex->__data.__lock = 0;
+  else
+    lll_unlock (mutex->__data.__lock, private);
+}
+
 int
 attribute_hidden
 __pthread_mutex_unlock_usercnt (pthread_mutex_t *mutex, int decr)
@@ -51,7 +66,7 @@ __pthread_mutex_unlock_usercnt (pthread_mutex_t *mutex, int decr)
 	--mutex->__data.__nusers;
 
       /* Unlock.  */
-      lll_unlock (mutex->__data.__lock, PTHREAD_MUTEX_PSHARED (mutex));
+      lll_mutex_unlock_optimized (mutex);
 
       LIBC_PROBE (mutex_release, 1, mutex);
 
-- 
2.30.2