[lttng-dev] [PATCH lttng-tools] Fix: thread exit vs futex wait/wakeup race

[Mathieu Desnoyers]

relayd_live_stop performs, in this order:

        CMM_STORE_SHARED(live_dispatch_thread_exit, 1);   [A]
        futex_nto1_wake(&viewer_conn_queue.futex);        [B]

whereas thread_dispatcher does:

   while (!CMM_LOAD_SHARED(live_dispatch_thread_exit)) {  [1]

     [...]
     futex_nto1_prepare(&viewer_conn_queue.futex);        [2]
     [...]
     futex_nto1_wait(&viewer_conn_queue.futex);           [3]

Unfortunately, on the following sequence:

[1] [A] [B] [2] [3]

thread_dispatcher will end up hanging.

We need to move the live_dispatch_thread_exit load between "prepare" and
"wait" to fix this.

There are similar scenarios with relay_thread_dispatcher, and the
session daemon thread_dispatch_ust_registration, which are also fixed
here.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
---
 src/bin/lttng-relayd/live.c   | 6 +++++-
 src/bin/lttng-relayd/main.c   | 6 +++++-
 src/bin/lttng-sessiond/main.c | 6 +++++-
 3 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/src/bin/lttng-relayd/live.c b/src/bin/lttng-relayd/live.c
index 61acafe..f00270b 100644
--- a/src/bin/lttng-relayd/live.c
+++ b/src/bin/lttng-relayd/live.c
@@ -654,12 +654,16 @@ void *thread_dispatcher(void *data)
 
 	health_code_update();
 
-	while (!CMM_LOAD_SHARED(live_dispatch_thread_exit)) {
+	for (;;) {
 		health_code_update();
 
 		/* Atomically prepare the queue futex */
 		futex_nto1_prepare(&viewer_conn_queue.futex);
 
+		if (CMM_LOAD_SHARED(live_dispatch_thread_exit)) {
+			break;
+		}
+
 		do {
 			health_code_update();
 
diff --git a/src/bin/lttng-relayd/main.c b/src/bin/lttng-relayd/main.c
index 2fcc60a..0eb8e28 100644
--- a/src/bin/lttng-relayd/main.c
+++ b/src/bin/lttng-relayd/main.c
@@ -977,12 +977,16 @@ static void *relay_thread_dispatcher(void *data)
 
 	health_code_update();
 
-	while (!CMM_LOAD_SHARED(dispatch_thread_exit)) {
+	for (;;) {
 		health_code_update();
 
 		/* Atomically prepare the queue futex */
 		futex_nto1_prepare(&relay_conn_queue.futex);
 
+		if (CMM_LOAD_SHARED(dispatch_thread_exit)) {
+			break;
+		}
+
 		do {
 			health_code_update();
 
diff --git a/src/bin/lttng-sessiond/main.c b/src/bin/lttng-sessiond/main.c
index 9b6f70c..e282a9c 100644
--- a/src/bin/lttng-sessiond/main.c
+++ b/src/bin/lttng-sessiond/main.c
@@ -1934,12 +1934,16 @@ static void *thread_dispatch_ust_registration(void *data)
 
 	DBG("[thread] Dispatch UST command started");
 
-	while (!CMM_LOAD_SHARED(dispatch_thread_exit)) {
+	for (;;) {
 		health_code_update();
 
 		/* Atomically prepare the queue futex */
 		futex_nto1_prepare(&ust_cmd_queue.futex);
 
+		if (CMM_LOAD_SHARED(dispatch_thread_exit)) {
+			break;
+		}
+
 		do {
 			struct ust_app *app = NULL;
 			ust_cmd = NULL;
-- 
2.1.4