Be more careful about barriers when releasing BackgroundWorkerSlots.

tglsfdc · tglsfdc · commit 5d195dc40af0 · 2021-05-15T12:21:06.000-04:00
ForgetBackgroundWorker lacked any memory barrier at all, while BackgroundWorkerStateChange had one but unaccountably did additional manipulation of the slot after the barrier. AFAICS, the rule must be that the barrier is immediately before setting or clearing slot->in_use. It looks like back in 9.6 when ForgetBackgroundWorker was first written, there might have been some case for not needing a barrier there, but I'm not very convinced of that --- the fact that the load of bgw_notify_pid is in the caller doesn't seem to guarantee no memory ordering problem. So patch 9.6 too. It's likely that this doesn't fix any observable bug on Intel hardware, but machines with weaker memory ordering rules could have problems here. Discussion: https://postgr.es/m/4046084.1620244003@sss.pgh.pa.us
diff --git a/src/backend/postmaster/bgworker.c b/src/backend/postmaster/bgworker.c
@@ -290,9 +290,11 @@ BackgroundWorkerStateChange(void)
 			 * bgw_notify_pid completes before the store to in_use.
 			 */
 			notify_pid = slot->worker.bgw_notify_pid;
-			pg_memory_barrier();
 			slot->pid = 0;
+
+			pg_memory_barrier();
 			slot->in_use = false;
+
 			if (notify_pid != 0)
 				kill(notify_pid, SIGUSR1);
 
@@ -378,6 +380,8 @@ BackgroundWorkerStateChange(void)
  * points to it.  This convention allows deletion of workers during
  * searches of the worker list, and saves having to search the list again.
  *
+ * Caller is responsible for notifying bgw_notify_pid, if appropriate.
+ *
  * This function must be invoked only in the postmaster.
  */
 void
@@ -390,6 +394,13 @@ ForgetBackgroundWorker(slist_mutable_iter *cur)
 
 	Assert(rw->rw_shmem_slot < max_worker_processes);
 	slot = &BackgroundWorkerData->slot[rw->rw_shmem_slot];
+	Assert(slot->in_use);
+
+	/*
+	 * This memory barrier might not be completely necessary, but let's be
+	 * sure.
+	 */
+	pg_memory_barrier();
 	slot->in_use = false;
 
 	ereport(DEBUG1,
