X-Git-Url: http://git.lttng.org/?a=blobdiff_plain;f=src%2Flib%2Flttng-ust%2Flttng-ust-comm.c;h=0fe2da466ad1cce241b0b82efd864e651223341e;hb=8e95eaadb72a1a97486f15cc128ff6ed2fcd0196;hp=384a6a582af8a2034c9678b31b400b4e33b95f50;hpb=ab64db3571ab2b39a2218f81b3483768f815822a;p=lttng-ust.git

diff --git a/src/lib/lttng-ust/lttng-ust-comm.c b/src/lib/lttng-ust/lttng-ust-comm.c
index 384a6a58..0fe2da46 100644
--- a/src/lib/lttng-ust/lttng-ust-comm.c
+++ b/src/lib/lttng-ust/lttng-ust-comm.c
@@ -118,6 +118,28 @@ static int lttng_ust_comm_should_quit;
  */
 int lttng_ust_loaded __attribute__((weak));
 
+/*
+ * Notes on async-signal-safety of ust lock: a few libc functions are used
+ * which are not strictly async-signal-safe:
+ *
+ * - pthread_setcancelstate
+ * - pthread_mutex_lock
+ * - pthread_mutex_unlock
+ *
+ * As of glibc 2.35, the implementation of pthread_setcancelstate only
+ * touches TLS data, and it appears to be safe to use from signal
+ * handlers. If the libc implementation changes, this will need to be
+ * revisited, and we may ask glibc to provide an async-signal-safe
+ * pthread_setcancelstate.
+ *
+ * As of glibc 2.35, the implementation of pthread_mutex_lock/unlock
+ * for fast mutexes only relies on the pthread_mutex_t structure.
+ * Disabling signals around all uses of this mutex ensures
+ * signal-safety. If the libc implementation changes and eventually uses
+ * other global resources, this will need to be revisited and we may
+ * need to implement our own mutex.
+ */
+
 /*
  * Return 0 on success, -1 if should quit.
  * The lock is taken in both cases.
@@ -365,7 +387,7 @@ const char *get_lttng_home_dir(void)
  * Force a read (imply TLS allocation for dlopen) of TLS variables.
  */
 static
-void lttng_nest_count_alloc_tls(void)
+void lttng_ust_nest_count_alloc_tls(void)
 {
 	asm volatile ("" : : "m" (URCU_TLS(lttng_ust_nest_count)));
 }
@@ -380,26 +402,26 @@ void lttng_ust_mutex_nest_alloc_tls(void)
  * Allocate lttng-ust urcu TLS.
  */
 static
-void lttng_lttng_ust_urcu_alloc_tls(void)
+void lttng_ust_urcu_alloc_tls(void)
 {
 	(void) lttng_ust_urcu_read_ongoing();
 }
 
-void lttng_ust_alloc_tls(void)
+void lttng_ust_common_init_thread(int flags)
 {
-	lttng_lttng_ust_urcu_alloc_tls();
+	lttng_ust_urcu_alloc_tls();
 	lttng_ringbuffer_alloc_tls();
-	lttng_vtid_alloc_tls();
-	lttng_nest_count_alloc_tls();
-	lttng_procname_alloc_tls();
+	lttng_ust_vtid_init_thread(flags);
+	lttng_ust_nest_count_alloc_tls();
+	lttng_ust_procname_init_thread(flags);
 	lttng_ust_mutex_nest_alloc_tls();
-	lttng_ust_perf_counter_alloc_tls();
+	lttng_ust_perf_counter_init_thread(flags);
 	lttng_ust_common_alloc_tls();
-	lttng_cgroup_ns_alloc_tls();
-	lttng_ipc_ns_alloc_tls();
-	lttng_net_ns_alloc_tls();
-	lttng_time_ns_alloc_tls();
-	lttng_uts_ns_alloc_tls();
+	lttng_ust_cgroup_ns_init_thread(flags);
+	lttng_ust_ipc_ns_init_thread(flags);
+	lttng_ust_net_ns_init_thread(flags);
+	lttng_ust_time_ns_init_thread(flags);
+	lttng_ust_uts_ns_init_thread(flags);
 	lttng_ust_ring_buffer_client_discard_alloc_tls();
 	lttng_ust_ring_buffer_client_discard_rt_alloc_tls();
 	lttng_ust_ring_buffer_client_overwrite_alloc_tls();
@@ -424,7 +446,9 @@ void lttng_ust_init_thread(void)
 	 * ensure those are initialized before a signal handler nesting over
 	 * this thread attempts to use them.
 	 */
-	lttng_ust_alloc_tls();
+	lttng_ust_common_init_thread(LTTNG_UST_INIT_THREAD_MASK);
+
+	lttng_ust_urcu_register_thread();
 }
 
 int lttng_get_notify_socket(void *owner)
@@ -1735,18 +1759,25 @@ void wait_for_sessiond(struct sock_info *sock_info)
 
 	DBG("Waiting for %s apps sessiond", sock_info->name);
 	/* Wait for futex wakeup */
-	if (uatomic_read((int32_t *) sock_info->wait_shm_mmap))
-		goto end_wait;
-
-	while (lttng_ust_futex_async((int32_t *) sock_info->wait_shm_mmap,
-			FUTEX_WAIT, 0, NULL, NULL, 0)) {
+	while (!uatomic_read((int32_t *) sock_info->wait_shm_mmap)) {
+		if (!lttng_ust_futex_async((int32_t *) sock_info->wait_shm_mmap, FUTEX_WAIT, 0, NULL, NULL, 0)) {
+			/*
+			 * Prior queued wakeups queued by unrelated code
+			 * using the same address can cause futex wait to
+			 * return 0 even through the futex value is still
+			 * 0 (spurious wakeups). Check the value again
+			 * in user-space to validate whether it really
+			 * differs from 0.
+			 */
+			continue;
+		}
 		switch (errno) {
-		case EWOULDBLOCK:
+		case EAGAIN:
 			/* Value already changed. */
 			goto end_wait;
 		case EINTR:
 			/* Retry if interrupted by signal. */
-			break;	/* Get out of switch. */
+			break;	/* Get out of switch. Check again. */
 		case EFAULT:
 			wait_poll_fallback = 1;
 			DBG(
@@ -1786,7 +1817,7 @@ void *ust_listener_thread(void *arg)
 	int sock, ret, prev_connect_failed = 0, has_waited = 0, fd;
 	long timeout;
 
-	lttng_ust_alloc_tls();
+	lttng_ust_common_init_thread(0);
 	/*
 	 * If available, add '-ust' to the end of this thread's
 	 * process name
@@ -2154,7 +2185,7 @@ void lttng_ust_ctor(void)
 	 * to be the dynamic linker mutex) and ust_lock, taken within
 	 * the ust lock.
 	 */
-	lttng_ust_alloc_tls();
+	lttng_ust_common_init_thread(0);
 
 	lttng_ust_loaded = 1;
 
@@ -2468,7 +2499,7 @@ void lttng_ust_before_fork(sigset_t *save_sigset)
 	int ret;
 
 	/* Allocate lttng-ust TLS. */
-	lttng_ust_alloc_tls();
+	lttng_ust_common_init_thread(0);
 
 	if (URCU_TLS(lttng_ust_nest_count))
 		return;