Revert "Add freebsd support for getcpu (use cpu nr 0)"

[lttng-ust.git] / liblttng-ust / lttng-ust-comm.c

diff --git a/liblttng-ust/lttng-ust-comm.c b/liblttng-ust/lttng-ust-comm.c
index dda4743fc240cf7c2dc7f71e15846a8ec2a691ca..86cce18d57fe658588db907ed8016f39e95f975f 100644 (file)
--- 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);
 }