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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 <ehoover@mines.edu>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=54671
Signed-off-by: Uli Schlachter <psychon@znc.in>
This commit is contained in:
Uli Schlachter 2012-09-24 22:07:51 +02:00
parent c16cc5467e
commit 23911a707b
1 changed files with 12 additions and 2 deletions
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14
src/xcb_conn.c
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@ -432,10 +432,20 @@ int _xcb_conn_wait(xcb_connection_t *c, pthread_cond_t *cond, struct iovec **vec
if(ret)
{
/* 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 USE_POLL
if((fd.revents & POLLIN) == POLLIN)
if(may_read && (fd.revents & POLLIN) == POLLIN)
#else
if(FD_ISSET(c->fd, &rfds))
if(may_read && FD_ISSET(c->fd, &rfds))
#endif
ret = ret && _xcb_in_read(c);