diff mbox series

[V2,02/13] USB: serial: keyspan_pda: Replace in_interrupt() usage

Message ID 20201019101109.753597069@linutronix.de
State Superseded
Headers show
Series USB: Cleanup in_interupt/in_irq/in_atomic() usage | expand

Commit Message

Thomas Gleixner Oct. 19, 2020, 10:06 a.m. UTC 
keyspan_pda_write() uses in_interrupt() to check whether it is safe to
invoke functions which might sleep.

The usage of in_interrupt() in drivers is phased out and Linus clearly
requested that code which changes behaviour depending on context should
either be seperated or the context be conveyed in an argument passed by the
caller, which usually knows the context.

Aside of that it does not cover all contexts which cannot sleep,
e.g. preempt disabled regions which cannot be reliably detected on all
kernel configurations.

With the current printk() implementation console->write() can be invoked
from almost any context. The upcoming rework of the console core will
provide thread context for console drivers which require to sleep.

For now, restrict the room query which can sleep to tty writes which happen
from preemptible task context.

The usability for dmesg output is limited anyway because it's almost
guaranteed to drop the 'LF' which is submitted after the dmesg line because
the device supports only one transfer on flight. Same for any printk()
which is coming in before the previous transfer has been done.

This new restriction does not make it worse than before, but it makes the
condition correct under all circumstances.

Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Johan Hovold <johan@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-usb@vger.kernel.org

---
 drivers/usb/serial/keyspan_pda.c |   10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

Comments

Johan Hovold Oct. 27, 2020, 8:16 a.m. UTC | #1 
On Mon, Oct 26, 2020 at 01:47:53PM +0100, Sebastian Andrzej Siewior wrote:
> On 2020-10-25 17:56:47 [+0100], Johan Hovold wrote:
> > There's a ton of issues with this driver, but this is arguably making
> > things worse. A line discipline may call write() from just about any
> > context so we cannot rely on tty being non-NULL here (e.g. PPP).
> 
> I wasn't aware of that. I've been looking at the callers each time a
> `tty' was passed it looked like a preemptible context (due to mutex /
> GFP_KERNEL) and so on.

Yeah, the default line discipline only calls in preemptible context
(these days), but others do not (e.g. see ppp_async_push()).

Johan
diff mbox series

Patch

--- a/drivers/usb/serial/keyspan_pda.c
+++ b/drivers/usb/serial/keyspan_pda.c
@@ -477,10 +477,12 @@  static int keyspan_pda_write(struct tty_
 
 	count = (count > port->bulk_out_size) ? port->bulk_out_size : count;
 
-	/* Check if we might overrun the Tx buffer.   If so, ask the
-	   device how much room it really has.  This is done only on
-	   scheduler time, since usb_control_msg() sleeps. */
-	if (count > priv->tx_room && !in_interrupt()) {
+	/*
+	 * Check if we might overrun the Tx buffer. If so, ask the device
+	 * how much room it really has. This can only be invoked for tty
+	 * usage because the console write can't sleep.
+	 */
+	if (count > priv->tx_room && tty) {
 		u8 *room;
 
 		room = kmalloc(1, GFP_KERNEL);