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authorPaul Mackerras <paulus@samba.org>2006-06-12 12:16:26 +1000
committerLinus Torvalds <torvalds@g5.osdl.org>2006-06-11 20:40:39 -0700
commit289a1e995e74734b5ec76ca8a5490058f4fecc24 (patch)
treeada89355755d9ff4f5f3b77ced8483643a52e8b6 /drivers/char/n_tty.c
parentdc4967e756021f318d125c9f4fa98b958ae9f2de (diff)
downloadlinux-linaro-stable-289a1e995e74734b5ec76ca8a5490058f4fecc24.tar.gz
[PATCH] Fix for the PPTP hangs that have been reported
People have been reporting that PPP connections over ptys, such as used with PPTP, will hang randomly when transferring large amounts of data, for instance in http://bugzilla.kernel.org/show_bug.cgi?id=6530. I have managed to reproduce the problem, and the patch below fixes the actual cause. The problem is not in fact in ppp_async.c but in n_tty.c. What happens is that when pptp reads from the pty, we call read_chan() in drivers/char/n_tty.c on the master side of the pty. That copies all the characters out of its buffer to userspace and then calls check_unthrottle(), which calls the pty unthrottle routine, which calls tty_wakeup on the slave side, which calls ppp_asynctty_wakeup, which calls tasklet_schedule. So far so good. Since we are in process context, the tasklet runs immediately and calls ppp_async_process(), which calls ppp_async_push, which calls the tty->driver->write function to send some more output. However, tty->driver->write() returns zero, because the master tty->receive_room is still zero. We haven't returned from check_unthrottle() yet, and read_chan() only updates tty->receive_room _after_ calling check_unthrottle. That means that the driver->write call in ppp_async_process() returns 0. That would be fine if we were going to get a subsequent wakeup call, but we aren't (we just had it, and the buffer is now empty). The solution is for n_tty.c to update tty->receive_room _before_ calling the driver unthrottle routine. The patch below does this. With this patch I was able to transfer a 900MB file over a PPTP connection (taking about 25 minutes), whereas without the patch the connection would always stall in under a minute. Signed-off-by: Paul Mackerras <paulus@samba.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Diffstat (limited to 'drivers/char/n_tty.c')
-rw-r--r--drivers/char/n_tty.c4
1 files changed, 3 insertions, 1 deletions
diff --git a/drivers/char/n_tty.c b/drivers/char/n_tty.c
index ede365d05387..b9371d5bf790 100644
--- a/drivers/char/n_tty.c
+++ b/drivers/char/n_tty.c
@@ -1384,8 +1384,10 @@ do_it_again:
* longer than TTY_THRESHOLD_UNTHROTTLE in canonical mode,
* we won't get any more characters.
*/
- if (n_tty_chars_in_buffer(tty) <= TTY_THRESHOLD_UNTHROTTLE)
+ if (n_tty_chars_in_buffer(tty) <= TTY_THRESHOLD_UNTHROTTLE) {
+ n_tty_set_room(tty);
check_unthrottle(tty);
+ }
if (b - buf >= minimum)
break;