do_tty_write() can block even with O_NONBLOCK?

do_tty_write() can block even with O_NONBLOCK?

[Dave Johnson]

I have two processes with the same tty open, one opens blocking and
one opens nonblocking.

If the blocking process blocks doing a write (due to flow control,
just going to fast, etc...) the nonblocking process will also block
when it writes until the blocking process unblocks.

This seems to occur because do_tty_write() isn't checking for
O_NONBLOCK when taking the tty's write mutex.


Signed-off-by: Dave Johnson <djohnson+linux-kernel@sw.starentnetworks.com>

===== drivers/char/tty_io.c 1.237 vs edited =====
--- 1.237/drivers/char/tty_io.c	2007-03-18 16:40:06 -04:00
+++ edited/drivers/char/tty_io.c	2007-05-01 10:53:27 -04:00
@@ -1690,9 +1690,13 @@
 	ssize_t ret = 0, written = 0;
 	unsigned int chunk;
 	
-	/* FIXME: O_NDELAY ... */
-	if (mutex_lock_interruptible(&tty->atomic_write_lock)) {
-		return -ERESTARTSYS;
+	if (file->f_flags & O_NONBLOCK) {
+		if (!mutex_trylock(&tty->atomic_write_lock))
+			return -EAGAIN;
+	}
+	else {
+		if (mutex_lock_interruptible(&tty->atomic_write_lock))
+			return -ERESTARTSYS;
 	}
 
 	/*

Re: do_tty_write() can block even with O_NONBLOCK?

[Alan Cox]

On Wed, 2 May 2007 11:17:00 -0400
Dave Johnson <djohnson+linux-kernel@sw.starentnetworks.com> wrote:

> 
> I have two processes with the same tty open, one opens blocking and
> one opens nonblocking.
> 
> If the blocking process blocks doing a write (due to flow control,
> just going to fast, etc...) the nonblocking process will also block
> when it writes until the blocking process unblocks.
> 
> This seems to occur because do_tty_write() isn't checking for
> O_NONBLOCK when taking the tty's write mutex.
> 
> 
> Signed-off-by: Dave Johnson <djohnson+linux-kernel@sw.starentnetworks.com>

Acked-by: Alan Cox <alan@redhat.com>

Looks fine to me.