pthread: Use private futexes by default in condition variables
Now that the system properly uses shared condvars when needed, we
can enable the use of private futexes for them too.
Change-Id: Icf8351fc0a2309f764cba45c65bc3af047720cdf
diff --git a/libc/bionic/pthread.c b/libc/bionic/pthread.c
index 6a2329f..f93a241 100644
--- a/libc/bionic/pthread.c
+++ b/libc/bionic/pthread.c
@@ -1361,9 +1361,11 @@
/* We use one bit in condition variable values as the 'shared' flag
* The rest is a counter.
*/
-#define COND_SHARING_MASK 0x0001
+#define COND_SHARED_MASK 0x0001
#define COND_COUNTER_INCREMENT 0x0002
-#define COND_COUNTER_MASK (~COND_SHARING_MASK)
+#define COND_COUNTER_MASK (~COND_SHARED_MASK)
+
+#define COND_IS_SHARED(c) (((c)->value & COND_SHARED_MASK) != 0)
/* XXX *technically* there is a race condition that could allow
* XXX a signal to be missed. If thread A is preempted in _wait()
@@ -1382,7 +1384,7 @@
cond->value = 0;
if (attr != NULL && *attr == PTHREAD_PROCESS_SHARED)
- cond->value |= COND_SHARING_MASK;
+ cond->value |= COND_SHARED_MASK;
return 0;
}
@@ -1397,13 +1399,19 @@
}
/* This function is used by pthread_cond_broadcast and
- * pthread_cond_signal to atomically decrement the counter.
+ * pthread_cond_signal to atomically decrement the counter
+ * then wake-up 'counter' threads.
*/
-static void
-__pthread_cond_pulse(pthread_cond_t *cond)
+static int
+__pthread_cond_pulse(pthread_cond_t *cond, int counter)
{
- long flags = (cond->value & ~COND_COUNTER_MASK);
+ long flags;
+ int wake_op;
+ if (__unlikely(cond == NULL))
+ return EINVAL;
+
+ flags = (cond->value & ~COND_COUNTER_MASK);
for (;;) {
long oldval = cond->value;
long newval = ((oldval - COND_COUNTER_INCREMENT) & COND_COUNTER_MASK)
@@ -1411,26 +1419,20 @@
if (__atomic_cmpxchg(oldval, newval, &cond->value) == 0)
break;
}
+
+ wake_op = COND_IS_SHARED(cond) ? FUTEX_WAKE : FUTEX_WAKE_PRIVATE;
+ __futex_syscall3(&cond->value, wake_op, counter);
+ return 0;
}
int pthread_cond_broadcast(pthread_cond_t *cond)
{
- if (__unlikely(cond == NULL))
- return EINVAL;
-
- __pthread_cond_pulse(cond);
- __futex_wake(&cond->value, INT_MAX);
- return 0;
+ return __pthread_cond_pulse(cond, INT_MAX);
}
int pthread_cond_signal(pthread_cond_t *cond)
{
- if (__unlikely(cond == NULL))
- return EINVAL;
-
- __pthread_cond_pulse(cond);
- __futex_wake(&cond->value, 1);
- return 0;
+ return __pthread_cond_pulse(cond, 1);
}
int pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex)
@@ -1444,9 +1446,10 @@
{
int status;
int oldvalue = cond->value;
+ int wait_op = COND_IS_SHARED(cond) ? FUTEX_WAIT : FUTEX_WAIT_PRIVATE;
pthread_mutex_unlock(mutex);
- status = __futex_wait(&cond->value, oldvalue, reltime);
+ status = __futex_syscall4(&cond->value, wait_op, oldvalue, reltime);
pthread_mutex_lock(mutex);
if (status == (-ETIMEDOUT)) return ETIMEDOUT;
diff --git a/libc/docs/CHANGES.TXT b/libc/docs/CHANGES.TXT
index 08b2009..9080685 100644
--- a/libc/docs/CHANGES.TXT
+++ b/libc/docs/CHANGES.TXT
@@ -83,13 +83,10 @@
- <pthread.h>: Add pthread_condattr_init/destroy/setpshared/getpshared functions
to enable proper shared conditional variable initialization.
- Modify the pthread_mutex_t implementation to use private futexes for
- performance reasons. Mutexes are no longer shareable between processes
- by default anymore, unless you use PTHREAD_PROCESS_SHARED with
- pthread_mutexattr_setpshared().
-
- XXX: The implementation still always uses shared condvars, independent
- of the flags being selected. This will change in a later commit.
+ Modify the pthread_mutex_t and pthread_cond_t implementation to use private
+ futexes for performance reasons. Mutexes and Condvars are no longer shareable
+ between processes by default anymore, unless you use PTHREAD_PROCESS_SHARED
+ with pthread_mutexattr_setpshared() and/or pthread_condattr_setpshared().
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Differences between Android 2.1 and 2.0.1: