X-Git-Url: http://git.lttng.org./?a=blobdiff_plain;f=libringbuffer%2Ffrontend_internal.h;h=a6b96c1a098ab089199cbc9d06f59549bcc4328f;hb=1c5e467e5da8d549fa499c3ceb9739c5f704260d;hp=02cb4d4a75d37c4b70f1f515be7fe5a1e18443d9;hpb=4746ae29409b78e96543a3b207c91a3c510c6476;p=lttng-ust.git

diff --git a/libringbuffer/frontend_internal.h b/libringbuffer/frontend_internal.h
index 02cb4d4a..a6b96c1a 100644
--- a/libringbuffer/frontend_internal.h
+++ b/libringbuffer/frontend_internal.h
@@ -376,7 +376,8 @@ void lib_ring_buffer_check_deliver(const struct lib_ring_buffer_config *config,
 			 * RING_BUFFER_WAKEUP_BY_WRITER wakeup is not lock-free.
 			 */
 			if (config->wakeup == RING_BUFFER_WAKEUP_BY_WRITER
-			    && uatomic_read(&buf->active_readers)
+			    && (uatomic_read(&buf->active_readers)
+				|| uatomic_read(&buf->active_shadow_readers))
 			    && lib_ring_buffer_poll_deliver(config, buf, chan, handle)) {
 				int wakeup_fd = shm_get_wakeup_fd(handle, &buf->self._ref);
 
@@ -386,6 +387,20 @@ void lib_ring_buffer_check_deliver(const struct lib_ring_buffer_config *config,
 					 * Wake-up the other end by
 					 * writing a null byte in the
 					 * pipe (non-blocking).
+					 * Important note: Because
+					 * writing into the pipe is
+					 * non-blocking (and therefore
+					 * we allow dropping wakeup
+					 * data, as long as there is
+					 * wakeup data present in the
+					 * pipe buffer to wake up the
+					 * consumer), the consumer
+					 * should perform the following
+					 * sequence for waiting:
+					 * 1) empty the pipe (reads).
+					 * 2) check if there is data in
+					 *    the buffer.
+					 * 3) wait on the pipe (poll).
 					 */
 					do {
 						ret = write(wakeup_fd, "", 1);