Speed up unnamed POSIX semaphores

When sem_wait() used its futexes it would always use process shared mode
which can be problematic on platforms like Windows, where that causes it
to use the slow futex polyfill. Now when sem_init() is called in private
mode that'll be passed along so we can use a faster WaitOnAddress() call

Author: jart

libc/thread/sem_destroy.c

@@ -18,6 +18,7 @@
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/assert.h"
 #include "libc/intrin/atomic.h"
+#include "libc/intrin/strace.h"
 #include "libc/limits.h"
 #include "libc/sysv/errfuns.h"
 #include "libc/thread/semaphore.h"
@@ -40,14 +41,20 @@
  * @raise EBUSY if `sem` has waiters
  */
 int sem_destroy(sem_t *sem) {
-  int waiters;
+  int rc, waiters;
   npassert(sem->sem_magic != SEM_MAGIC_NAMED);
-  if (sem->sem_magic != SEM_MAGIC_UNNAMED)
-    return einval();
-  waiters = atomic_load_explicit(&sem->sem_waiters, memory_order_relaxed);
-  unassert(waiters >= 0);
-  if (waiters)
-    return ebusy();
-  atomic_store_explicit(&sem->sem_value, INT_MIN, memory_order_relaxed);
-  return 0;
+  if (sem->sem_magic != SEM_MAGIC_UNNAMED) {
+    rc = einval();
+  } else {
+    waiters = atomic_load_explicit(&sem->sem_waiters, memory_order_relaxed);
+    unassert(waiters >= 0);
+    if (waiters) {
+      rc = ebusy();
+    } else {
+      atomic_store_explicit(&sem->sem_value, INT_MIN, memory_order_relaxed);
+      rc = 0;
+    }
+  }
+  STRACE("sem_destroy(%p) → %d% m", sem, rc);
+  return rc;
 }

libc/thread/sem_init.c

@@ -19,6 +19,7 @@
 #include "libc/calls/calls.h"
 #include "libc/dce.h"
 #include "libc/intrin/atomic.h"
+#include "libc/intrin/strace.h"
 #include "libc/limits.h"
 #include "libc/sysv/errfuns.h"
 #include "libc/thread/semaphore.h"
@@ -37,12 +38,17 @@
  * @raise EINVAL if `value` exceeds `SEM_VALUE_MAX`
  */
 int sem_init(sem_t *sem, int pshared, unsigned value) {
-  if (value > SEM_VALUE_MAX)
-    return einval();
-  sem->sem_magic = SEM_MAGIC_UNNAMED;
-  atomic_store_explicit(&sem->sem_value, value, memory_order_relaxed);
-  sem->sem_pshared = !!pshared;
-  sem->sem_pid = getpid();
-  sem->sem_waiters = 0;
-  return 0;
+  int rc;
+  if (value > SEM_VALUE_MAX) {
+    rc = einval();
+  } else {
+    sem->sem_magic = SEM_MAGIC_UNNAMED;
+    atomic_store_explicit(&sem->sem_value, value, memory_order_relaxed);
+    sem->sem_pshared = !!pshared;
+    sem->sem_pid = getpid();
+    sem->sem_waiters = 0;
+    rc = 0;
+  }
+  STRACE("sem_init(%p, %hhhd, %u) → %d% m", sem, pshared, value, rc);
+  return rc;
 }

libc/thread/sem_post.c

@@ -46,7 +46,7 @@ int sem_post(sem_t *sem) {
   old = atomic_fetch_add_explicit(&sem->sem_value, 1, memory_order_acq_rel);
   unassert(old > INT_MIN);
   if (old >= 0) {
-    wakeups = nsync_futex_wake_(&sem->sem_value, 1, true);
+    wakeups = nsync_futex_wake_(&sem->sem_value, 1, sem->sem_pshared);
     npassert(wakeups >= 0);
     rc = 0;
   } else {

libc/thread/sem_timedwait.c

@@ -59,7 +59,7 @@ static void sem_timedwait_cleanup(void *arg) {
  * @cancelationpoint
  */
 int sem_timedwait(sem_t *sem, const struct timespec *abstime) {
-  int i, v, rc, e = errno;
+  int v, rc, e = errno;
 
 #if 0
   if (IsXnuSilicon() && sem->sem_magic == SEM_MAGIC_KERNEL) {
@@ -103,16 +103,13 @@ int sem_timedwait(sem_t *sem, const struct timespec *abstime) {
   }
 #endif
 
-  for (i = 0; i < 7; ++i) {
-    rc = sem_trywait(sem);
-    if (!rc) {
-      return rc;
-    } else if (errno == EAGAIN) {
-      errno = e;
-      sem_delay(i);
-    } else {
-      return rc;
-    }
+  rc = sem_trywait(sem);
+  if (!rc) {
+    return rc;
+  } else if (errno == EAGAIN) {
+    errno = e;
+  } else {
+    return rc;
   }
 
   BEGIN_CANCELATION_POINT;
@@ -122,7 +119,8 @@ int sem_timedwait(sem_t *sem, const struct timespec *abstime) {
 
   do {
     if (!(v = atomic_load_explicit(&sem->sem_value, memory_order_relaxed))) {
-      rc = nsync_futex_wait_(&sem->sem_value, v, true, CLOCK_REALTIME, abstime);
+      rc = nsync_futex_wait_(&sem->sem_value, v, sem->sem_pshared,
+                             CLOCK_REALTIME, abstime);
       if (rc == -EINTR || rc == -ECANCELED) {
         errno = -rc;
         rc = -1;

test/posix/unnamed_semaphore_test.c

@@ -0,0 +1,35 @@
+#include <pthread.h>
+#include <semaphore.h>
+
+#define THREADS    10
+#define ITERATIONS 100000
+
+int g_count;
+sem_t g_sem;
+
+void *worker(void *arg) {
+  for (int i = 0; i < ITERATIONS; ++i) {
+    if (sem_wait(&g_sem))
+      exit(6);
+    ++g_count;
+    if (sem_post(&g_sem))
+      exit(7);
+  }
+  return 0;
+}
+
+int main(int argc, char *argv[]) {
+  pthread_t th[THREADS];
+  if (sem_init(&g_sem, 0, 1))
+    return 1;
+  for (int i = 0; i < THREADS; ++i)
+    if (pthread_create(&th[i], 0, worker, 0))
+      return 2;
+  for (int i = 0; i < THREADS; ++i)
+    if (pthread_join(th[i], 0))
+      return 3;
+  if (g_count != THREADS * ITERATIONS)
+    return 4;
+  if (sem_destroy(&g_sem))
+    return 5;
+}