diff --git a/modules/libcom/src/osi/os/posix/osdThread.c b/modules/libcom/src/osi/os/posix/osdThread.c
index 17dfacbc86..9ed58db2a5 100644
--- a/modules/libcom/src/osi/os/posix/osdThread.c
+++ b/modules/libcom/src/osi/os/posix/osdThread.c
@@ -689,32 +689,45 @@ void epicsThreadMustJoin(epicsThreadId id)
 {
     void *ret = NULL;
     int status;
+    int prev;
+    epicsThreadId self;
 
-    if(!id) {
+    if(!id)
         return;
-    } else if(epicsAtomicCmpAndSwapIntT(&id->joinable, 1, 0)!=1) {
-        if(epicsThreadGetIdSelf()==id) {
-            errlogPrintf("Warning: %s thread self-join of unjoinable\n", id->name);
+
+    prev = epicsAtomicCmpAndSwapIntT(&id->joinable, 1, 0);
+    self = epicsThreadGetIdSelf();
+
+    if(prev==0) {
+        /* join of unjoinable */
+        if(self==id) {
+            errlogPrintf(ERL_WARNING ": %s thread self-join of unjoinable\n", id->name);
+            return;
 
         } else {
             /* try to error nicely, however in all likelihood de-ref of
              * 'id' has already caused SIGSEGV as we are racing thread exit,
              * which free's 'id'.
              */
-            cantProceed("Error: %s thread not joinable.\n", id->name);
+            cantProceed(ERL_ERROR ": %s can't join unjoinable %s\n", self->name, id->name);
         }
-        return;
-    }
 
-    status = pthread_join(id->tid, &ret);
-    if(status == EDEADLK) {
+    } else if(prev!=1) { /* also not 0, the only other allowed value. */
+        cantProceed(ERL_ERROR ": %s joins corrupt thread handle\n", self->name);
+
+    } else if(self==id) {
         /* Thread can't join itself (directly or indirectly)
          * so we detach instead.
          */
         status = pthread_detach(id->tid);
         checkStatusOnce(status, "pthread_detach");
-    } else checkStatusOnce(status, "pthread_join");
-    free_threadInfo(id);
+
+    } else {
+        status = pthread_join(id->tid, &ret);
+        checkStatusOnce(status, "pthread_join");
+    }
+
+    free_threadInfo(id); /* release joinable reference */
 }
 
 LIBCOM_API void epicsStdCall epicsThreadSuspendSelf(void)