2 files changed, 25 insertions, 9 deletions

diff --git a/src/backend/storage/ipc/shm_mq.c b/src/backend/storage/ipc/shm_mq.c
index 540f604c311..9e726eb628e 100644
--- a/src/backend/storage/ipc/shm_mq.c
+++ b/src/backend/storage/ipc/shm_mq.c
@@ -763,11 +763,11 @@ shm_mq_send_bytes(shm_mq_handle *mqh, Size nbytes, void *data, bool nowait,
 			 */
 			WaitLatch(&MyProc->procLatch, WL_LATCH_SET, 0);
 
-			/* An interrupt may have occurred while we were waiting. */
-			CHECK_FOR_INTERRUPTS();
-
 			/* Reset the latch so we don't spin. */
 			ResetLatch(&MyProc->procLatch);
+
+			/* An interrupt may have occurred while we were waiting. */
+			CHECK_FOR_INTERRUPTS();
 		}
 		else
 		{
@@ -860,11 +860,11 @@ shm_mq_receive_bytes(shm_mq *mq, Size bytes_needed, bool nowait,
 		 */
 		WaitLatch(&MyProc->procLatch, WL_LATCH_SET, 0);
 
-		/* An interrupt may have occurred while we were waiting. */
-		CHECK_FOR_INTERRUPTS();
-
 		/* Reset the latch so we don't spin. */
 		ResetLatch(&MyProc->procLatch);
+
+		/* An interrupt may have occurred while we were waiting. */
+		CHECK_FOR_INTERRUPTS();
 	}
 }
 
@@ -966,11 +966,11 @@ shm_mq_wait_internal(volatile shm_mq *mq, PGPROC *volatile * ptr,
 			/* Wait to be signalled. */
 			WaitLatch(&MyProc->procLatch, WL_LATCH_SET, 0);
 
-			/* An interrupt may have occurred while we were waiting. */
-			CHECK_FOR_INTERRUPTS();
-
 			/* Reset the latch so we don't spin. */
 			ResetLatch(&MyProc->procLatch);
+
+			/* An interrupt may have occurred while we were waiting. */
+			CHECK_FOR_INTERRUPTS();
 		}
 	}
 	PG_CATCH();
diff --git a/src/include/storage/latch.h b/src/include/storage/latch.h
index ab722e75662..fbd9e574566 100644
--- a/src/include/storage/latch.h
+++ b/src/include/storage/latch.h
@@ -52,6 +52,22 @@
  * do. Otherwise, if someone sets the latch between the check and the
  * ResetLatch call, you will miss it and Wait will incorrectly block.
  *
+ * Another valid coding pattern looks like:
+ *
+ * for (;;)
+ * {
+ *	   if (work to do)
+ *		   Do Stuff(); // in particular, exit loop if some condition satisfied
+ *	   WaitLatch();
+ *	   ResetLatch();
+ * }
+ *
+ * This is useful to reduce latch traffic if it's expected that the loop's
+ * termination condition will often be satisfied in the first iteration;
+ * the cost is an extra loop iteration before blocking when it is not.
+ * What must be avoided is placing any checks for asynchronous events after
+ * WaitLatch and before ResetLatch, as that creates a race condition.
+ *
  * To wake up the waiter, you must first set a global flag or something
  * else that the wait loop tests in the "if (work to do)" part, and call
  * SetLatch *after* that. SetLatch is designed to return quickly if the