|author||Uli Schlachter <firstname.lastname@example.org>||2012-09-24 20:07:51 (GMT)|
|committer||Uli Schlachter <email@example.com>||2012-09-30 08:10:18 (GMT)|
Fix a multi-thread deadlock
This fixes a deadlock which was seen in-the-wild with wine. It could happen that two threads tried to read from the socket at the same time and one of the thread got stuck inside of poll()/select(). The fix works by making sure that the writing thread doesn't steal the reading thread's reply. Debugged-by: Erich Hoover <firstname.lastname@example.org> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=54671 Signed-off-by: Uli Schlachter <email@example.com>
1 files changed, 12 insertions, 2 deletions
diff --git a/src/xcb_conn.c b/src/xcb_conn.c
index e01d566..6a7a806 100644
@@ -432,10 +432,20 @@ int _xcb_conn_wait(xcb_connection_t *c, pthread_cond_t *cond, struct iovec **vec
+ /* The code allows two threads to call select()/poll() at the same time.
+ * First thread just wants to read, a second thread wants to write, too.
+ * We have to make sure that we don't steal the reading thread's reply
+ * and let it get stuck in select()/poll().
+ * So a thread may read if either:
+ * - There is no other thread that wants to read (the above situation
+ * did not occur).
+ * - It is the reading thread (above situation occurred).
+ int may_read = c->in.reading == 1 || !count;
- if((fd.revents & POLLIN) == POLLIN)
+ if(may_read && (fd.revents & POLLIN) == POLLIN)
- if(FD_ISSET(c->fd, &rfds))
+ if(may_read && FD_ISSET(c->fd, &rfds))
ret = ret && _xcb_in_read(c);