From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Wed, 17 May 2017 22:36:54 +0000 (-0400)
Subject: Fix: thread exit vs futex wait/wakeup race
X-Git-Tag: v2.11.0-rc1~577
X-Git-Url: https://git.lttng.org/?a=commitdiff_plain;h=0ed3b1a8426260c2b8a9cb7987b6b242568a2d02;hp=0ed3b1a8426260c2b8a9cb7987b6b242568a2d02;p=lttng-tools.git

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>
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