X-Git-Url: http://git.liburcu.org/?a=blobdiff_plain;f=liblttng-ust%2Flttng-ust-comm.c;h=86cce18d57fe658588db907ed8016f39e95f975f;hb=efe0de097e8e1cd372a7eb926e0ec68b97e6ee50;hp=dda4743fc240cf7c2dc7f71e15846a8ec2a691ca;hpb=982052994af40833768fef0bd342a42a1775e572;p=lttng-ust.git

diff --git a/liblttng-ust/lttng-ust-comm.c b/liblttng-ust/lttng-ust-comm.c
index dda4743f..86cce18d 100644
--- a/liblttng-ust/lttng-ust-comm.c
+++ b/liblttng-ust/lttng-ust-comm.c
@@ -403,15 +403,10 @@ void cleanup_sock_info(struct sock_info *sock_info, int exiting)
 	}
 	sock_info->constructor_sem_posted = 0;
 	/*
-	 * When called from process exit, we allow this memory map to be
-	 * released by the OS at exit(), because removing it prior to
-	 * this can cause a segmentation fault when using the
-	 * futex_async timer-based fallback. And we cannot join those
-	 * threads because sys_futex does not react to the cancellation
-	 * request.
-	 *
-	 * So we actually _do_ release it only after a fork, since all
-	 * threads have vanished anyway.
+	 * wait_shm_mmap is used by listener threads outside of the
+	 * ust lock, so we cannot tear it down ourselves, because we
+	 * cannot join on these threads. Leave this task to the OS
+	 * process exit.
 	 */
 	if (!exiting && sock_info->wait_shm_mmap) {
 		ret = munmap(sock_info->wait_shm_mmap, sysconf(_SC_PAGE_SIZE));
@@ -891,6 +886,13 @@ void lttng_ust_cleanup(int exiting)
 	if (local_apps.allowed) {
 		cleanup_sock_info(&local_apps, exiting);
 	}
+	/*
+	 * The teardown in this function all affect data structures
+	 * accessed under the UST lock by the listener thread. This
+	 * lock, along with the lttng_ust_comm_should_quit flag, ensure
+	 * that none of these threads are accessing this data at this
+	 * point.
+	 */
 	lttng_ust_abi_exit();
 	lttng_ust_events_exit();
 	ltt_ring_buffer_client_discard_exit();
@@ -924,6 +926,7 @@ void __attribute__((destructor)) lttng_ust_exit(void)
 	lttng_ust_comm_should_quit = 1;
 	ust_unlock();
 
+	/* cancel threads */
 	ret = pthread_cancel(global_apps.ust_listener);
 	if (ret) {
 		ERR("Error cancelling global ust listener thread");
@@ -935,8 +938,12 @@ void __attribute__((destructor)) lttng_ust_exit(void)
 		}
 	}
 	/*
-	 * We cannot join the threads because they might be waiting on
-	 * sys_futex. Simply let the OS exit() clean up those threads.
+	 * Do NOT join threads: use of sys_futex makes it impossible to
+	 * join the threads without using async-cancel, but async-cancel
+	 * is delivered by a signal, which could hit the target thread
+	 * anywhere in its code path, including while the ust_lock() is
+	 * held, causing a deadlock for the other thread. Let the OS
+	 * cleanup the threads if there are stalled in a syscall.
 	 */
 	lttng_ust_cleanup(1);
 }