Fix: thread exit vs futex wait/wakeup race
authorMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Wed, 17 May 2017 22:36:54 +0000 (18:36 -0400)
committerJérémie Galarneau <jeremie.galarneau@efficios.com>
Fri, 19 May 2017 20:45:44 +0000 (16:45 -0400)
commit5f7d3a72d099c2b62a4116e313a700a0ab8906ce
tree6fa843ce6670ed2e5c0703cc4436fe1b405e986d
parent8f5e7d409e37a759daae12a9412e196d99140cef
Fix: thread exit vs futex wait/wakeup race

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@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
src/bin/lttng-relayd/live.c
src/bin/lttng-relayd/main.c
src/bin/lttng-sessiond/main.c
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