posix: optimize epicsThreadOnce()

Use atomic ops to short circuit when already initialized
mdavidsaver · Jan 10, 2023 · 77463b0 · 77463b0
1 parent 4da03c4
commit 77463b0
Showing 1 changed file with 18 additions and 9 deletions.
diff --git a/modules/libcom/src/osi/os/posix/osdThread.c b/modules/libcom/src/osi/os/posix/osdThread.c
@@ -523,33 +523,42 @@ LIBCOM_API unsigned int epicsStdCall epicsThreadGetStackSize (epicsThreadStackSi
 
 LIBCOM_API void epicsStdCall epicsThreadOnce(epicsThreadOnceId *id, void (*func)(void *), void *arg)
 {
-    static struct epicsThreadOSD threadOnceComplete;
-    #define EPICS_THREAD_ONCE_DONE &threadOnceComplete
+    static const struct epicsThreadOSD threadOnceComplete;
+    #define EPICS_THREAD_ONCE_DONE (void*)&threadOnceComplete
     int status;
+    void *prev, *self;
 
-    epicsThreadInit();
+    if(epicsAtomicGetPtrT((void**)id) == EPICS_THREAD_ONCE_DONE) {
+        return; /* fast path.  global init already done.  No need to lock */
+    }
+
+    self = epicsThreadGetIdSelf();
     status = mutexLock(&onceLock);
     if(status) {
         fprintf(stderr,"epicsThreadOnce: pthread_mutex_lock returned %s.\n",
             strerror(status));
         exit(-1);
     }
 
-    if (*id != EPICS_THREAD_ONCE_DONE) {
-        if (*id == EPICS_THREAD_ONCE_INIT) { /* first call */
-            *id = epicsThreadGetIdSelf();    /* mark active */
+    /* slow path, check again and attempt to begin */
+    prev = epicsAtomicCmpAndSwapPtrT((void**)id, EPICS_THREAD_ONCE_INIT, self);
+
+    if (prev != EPICS_THREAD_ONCE_DONE) {
+        if (prev == EPICS_THREAD_ONCE_INIT) { /* first call, already marked active */
             status = pthread_mutex_unlock(&onceLock);
             checkStatusQuit(status,"pthread_mutex_unlock", "epicsThreadOnce");
             func(arg);
             status = mutexLock(&onceLock);
             checkStatusQuit(status,"pthread_mutex_lock", "epicsThreadOnce");
-            *id = EPICS_THREAD_ONCE_DONE;    /* mark done */
-        } else if (*id == epicsThreadGetIdSelf()) {
+            epicsAtomicSetPtrT((void**)id, EPICS_THREAD_ONCE_DONE);    /* mark done */
+
+        } else if (prev == self) {
             status = pthread_mutex_unlock(&onceLock);
             checkStatusQuit(status,"pthread_mutex_unlock", "epicsThreadOnce");
             cantProceed("Recursive epicsThreadOnce() initialization\n");
+
         } else
-            while (*id != EPICS_THREAD_ONCE_DONE) {
+            while ((prev = epicsAtomicGetPtrT((void**)id)) != EPICS_THREAD_ONCE_DONE) {
                 /* Another thread is in the above func(arg) call. */
                 status = pthread_mutex_unlock(&onceLock);
                 checkStatusQuit(status,"pthread_mutex_unlock", "epicsThreadOnce");