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* [glibc] nptl: pthread_mutex_lock, pthread_mutex_unock single-threaded optimization
@ 2021-04-21 18:27 Florian Weimer
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From: Florian Weimer @ 2021-04-21 18:27 UTC (permalink / raw)
To: glibc-cvs
https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=99f841c441feeaa9a3d97fd91bb3d6ec8073c982
commit 99f841c441feeaa9a3d97fd91bb3d6ec8073c982
Author: Florian Weimer <fweimer@redhat.com>
Date: Wed Apr 21 19:49:51 2021 +0200
nptl: pthread_mutex_lock, pthread_mutex_unock single-threaded optimization
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>
Diff:
---
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);
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