From 5f635ec1e3ba70de2e8063f6f1a6163157220033 Mon Sep 17 00:00:00 2001 From: Uli Schlachter Date: Thu, 25 Aug 2011 14:18:16 +0200 Subject: [PATCH] Fix a dead-lock due to xcb_poll_for_reply Imagine two threads: Thread#1: for(;;) { xcb_get_input_focus_reply(c, xcb_get_input_focus(c), 0); } Thread#2: for(;;) { xcb_poll_for_event(c); } Since xcb_poll_for_event() calls _xcb_in_read() directly without synchronizing with any other readers, this causes two threads to end up calling recv() at the same time. We now have a race because any of these two threads could get read the GetInputFocus reply. If thread#2 reads this reply, it will be put in the appropriate queue and thread#1 will still be stuck in recv(), although its reply was already received. If no other reply or event causes this thread to wake up, the process deadlocks. To fix this, we have to make sure that there is only ever one thread reading from the connection. The obvious solution is to check in poll_for_next_event() if another thread is already reading (in which case c->in.reading != 0) and not to read from the wire in this case. This solution is actually correct if we assume that the other thread is blocked in poll() which means there isn't any data which can be read. Since we already checked that there is no event in the queue this means that poll_for_next_event() didn't find any event to return. There might be a small race here where the other thread already determined that there is data to read, but it still has to wait for c->iolock. However, this means that the next poll_for_next_event() will be able to read the event, so this shouldn't cause any problems. Signed-off-by: Uli Schlachter --- src/xcb_in.c | 2 +- 1 files changed, 1 insertions(+), 1 deletions(-) diff --git a/src/xcb_in.c b/src/xcb_in.c index 4acc3a2..e075a40 100644 --- a/src/xcb_in.c +++ b/src/xcb_in.c @@ -535,7 +535,7 @@ static xcb_generic_event_t *poll_for_next_event(xcb_connection_t *c, int queued) pthread_mutex_lock(&c->iolock); /* FIXME: follow X meets Z architecture changes. */ ret = get_event(c); - if(!ret && !queued && _xcb_in_read(c)) /* _xcb_in_read shuts down the connection on error */ + if(!ret && !queued && c->in.reading == 0 && _xcb_in_read(c)) /* _xcb_in_read shuts down the connection on error */ ret = get_event(c); pthread_mutex_unlock(&c->iolock); } -- 1.7.5.4