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Fix a deadlock related to event-base notification. Diagnosed by Zhou Li, Avi Bab, and Scott Lamb.

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The problem was that the thread doing the notification could block on
write in evthread_notify_base_default while holding the th_base_lock.
The main thread would never drain th_notify_fd[0], since it would need
th_base_lock to actually trigger events.
This commit is contained in:
Nick Mathewson 2010-07-05 13:17:47 -04:00
parent d89fdba424
commit 17522d2af8
1 changed files with 9 additions and 4 deletions
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13
event.c
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@ -1835,7 +1835,7 @@ evthread_notify_base_default(struct event_base *base)
#else
r = write(base->th_notify_fd[1], buf, 1);
#endif
return (r < 0) ? -1 : 0;
return (r < 0 && errno != EAGAIN) ? -1 : 0;
}
#if defined(_EVENT_HAVE_EVENTFD) && defined(_EVENT_HAVE_SYS_EVENTFD_H)
@ -2594,10 +2594,15 @@ evthread_make_base_notifiable(struct event_base *base)
base->th_notify_fn = notify;
/*
This can't be right, can it? We want writes to this socket to
just succeed.
evutil_make_socket_nonblocking(base->th_notify_fd[1]);
Making the second socket nonblocking is a bit subtle, given that we
ignore any EAGAIN returns when writing to it, and you don't usally
do that for a nonblocking socket. But if the kernel gives us EAGAIN,
then there's no need to add any more data to the buffer, since
the main thread is already either about to wake up and drain it,
or woken up and in the process of draining it.
*/
if (base->th_notify_fd[1] > 0)
evutil_make_socket_nonblocking(base->th_notify_fd[1]);
/* prepare an event that we can use for wakeup */
event_assign(&base->th_notify, base, base->th_notify_fd[0],