Fix: Disable cancellation around fd tracker lock

When using the ust fd tracker LD_PRELOAD library (liblttng-ust-fd.so),
cancelling other threads while they issue "close()" leads to deadlocks.

Fixes: #1201
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/liblttng-ust-comm/lttng-ust-fd-tracker.c b/liblttng-ust-comm/lttng-ust-fd-tracker.c
index 282895911c65569c6a5050bb859b9e73383c1a5a..1118163a201af456ac6ed28d490e09f458af08ef 100644 (file)
--- a/liblttng-ust-comm/lttng-ust-fd-tracker.c
+++ b/liblttng-ust-comm/lttng-ust-fd-tracker.c
@@ -66,6 +66,13 @@
  * is also held across fork.
  */
 static pthread_mutex_t ust_safe_guard_fd_mutex = PTHREAD_MUTEX_INITIALIZER;
+
+/*
+ * Cancel state when grabbing the ust_safe_guard_fd_mutex. Saved when
+ * locking, restored on unlock. Protected by ust_safe_guard_fd_mutex.
+ */
+static int ust_safe_guard_saved_cancelstate;
+
 /*
  * Track whether we are within lttng-ust or application, for close
  * system call override by LD_PRELOAD library.
@@ -127,6 +134,12 @@ void lttng_ust_init_fd_tracker(void)
 
 void lttng_ust_lock_fd_tracker(void)
 {
+       int ret, oldstate;
+
+       ret = pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &oldstate);
+       if (ret) {
+               ERR("pthread_setcancelstate: %s", strerror(ret));
+       }
        URCU_TLS(thread_fd_tracking) = 1;
        /*
         * Ensure the compiler don't move the store after the close()
@@ -134,10 +147,14 @@ void lttng_ust_lock_fd_tracker(void)
         */
        cmm_barrier();
        pthread_mutex_lock(&ust_safe_guard_fd_mutex);
+       ust_safe_guard_saved_cancelstate = oldstate;
 }
 
 void lttng_ust_unlock_fd_tracker(void)
 {
+       int ret, newstate, oldstate;
+
+       newstate = ust_safe_guard_saved_cancelstate;
        pthread_mutex_unlock(&ust_safe_guard_fd_mutex);
        /*
         * Ensure the compiler don't move the store before the close()
@@ -145,6 +162,10 @@ void lttng_ust_unlock_fd_tracker(void)
         */
        cmm_barrier();
        URCU_TLS(thread_fd_tracking) = 0;
+       ret = pthread_setcancelstate(newstate, &oldstate);
+       if (ret) {
+               ERR("pthread_setcancelstate: %s", strerror(ret));
+       }
 }
 
 static int dup_std_fd(int fd)