Fix: consumer fd recv thread should write into non-blocking pipe

[lttng-tools.git] / src / common / ust-consumer / ust-consumer.c

diff --git a/src/common/ust-consumer/ust-consumer.c b/src/common/ust-consumer/ust-consumer.c
index bfdb7e968fe9aa77ba9ad922e8fb6bf8748f4353..2b55fd4637ead2b056fcafecc47ef5504d7c5df5 100644 (file)
--- a/src/common/ust-consumer/ust-consumer.c
+++ b/src/common/ust-consumer/ust-consumer.c
@@ -257,11 +257,20 @@ int lttng_ustconsumer_recv_cmd(struct lttng_consumer_local_data *ctx,
                break;
        }
 end:
-       /* signal the poll thread */
-       ret = write(ctx->consumer_poll_pipe[1], "4", 1);
-       if (ret < 0) {
-               PERROR("write consumer poll");
-       }
+       /*
+        * 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 other end), the other end should
+        * perform the following sequence for waiting:
+        * 1) empty the pipe (reads).
+        * 2) perform update operation.
+        * 3) wait on the pipe (poll).
+        */
+       do {
+               ret = write(ctx->consumer_poll_pipe[1], "", 1);
+       } while (ret == -1UL && errno == EINTR);
 end_nosignal:
        return 0;
 }