Hi,
My DirectFB application kept hanging on shutdown whenever I used the
Linux Input driver, so I did some investigation to figure out why.
Here's what happens:
1. On shutdown, the threads processing /dev/event/X are all pthread_cancel()'ed.
2. For some reason[1] one or more of the threads reach the D_PERROR
("linux_input thread died\n") at the end of linux_input_EventThread().
3. One of the threads acquires the log->lock mutex in
direct_log_printf(), writes to log->fd, and then dies before releasing
the lock as write() is a cancellation point.
4. The next thread that tries to write to the log gets stuck on log->lock.
Here's my proposed fix, which temporarily changes the cancellation
state of the thread inside direct_log_printf() to prevent it from
being canceled while holding the lock (I'm by means a Pthreads guru,
so there might very well be a better solution):
--- a/lib/direct/log.c 2010-11-15 22:12:08.000000000 +0100
+++ b/lib/direct/log.c 2011-03-24 17:58:38.259808355 +0100
@@ -167,14 +167,19 @@
else {
int len;
char buf[512];
+ int old_cancellation_state;
len = vsnprintf( buf, sizeof(buf), format, args );
- pthread_mutex_lock( &log->lock );
+ /* Ensure the thread does not get canceled at the write(), which
+ * would prevent the log lock from being released. */
+ pthread_setcancelstate( PTHREAD_CANCEL_DISABLE,
&old_cancellation_state );
+ pthread_mutex_lock( &log->lock );
write( log->fd, buf, len );
-
pthread_mutex_unlock( &log->lock );
+
+ pthread_setcancelstate( old_cancellation_state, NULL );
}
va_end( args );
With the above patch my application no longer hangs on shutdown.
You probably also ought to make sure a thread can never die inside a
direct_log_lock()/direct_log_unlock() pair.
[1] This seems to be due to a uClibc bug that causes the select() on
linux_input.c:902 (DirectFB 1.4.11) to return -1 while errno remains
0. Seems to depend on subtle details in how the application was
compiled.
/Ulf
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