2007-05-02 15:46:35

by Dave Johnson

Subject: do_tty_write() can block even with O_NONBLOCK?

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 <[email protected]>

===== 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;
}

/*

2007-05-02 15:59:13

by Alan

Subject: Re: do_tty_write() can block even with O_NONBLOCK?

On Wed, 2 May 2007 11:17:00 -0400
Dave Johnson <[email protected]> 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 <[email protected]>

Acked-by: Alan Cox <[email protected]>

Looks fine to me.