From: David Brownell <dbrownell@xxxxxxxxxxxxxxxxxxxxx>
Fix regression to the serial gadget code created in January (2.6.29-rc1)
by the "n_tty: Fix loss of echoed characters and remove bkl from n_tty"
patch.
That patch made an incompatible change to the locking policy, so that
combining (a) tty->low_latency with (b) tty_flip_buffer_push() calls
from tasklets now generates lockdep warnings of the "you can't grab a
mutex from a tasklet" flavor.
This fix just disables the low_latency usage in this code, and gets rid
of that lockdep warning. Driver was *NOT* retested under RX stress loads
(which ISTR were the original motivation for kicking in low_latency).
Signed-off-by: David Brownell <dbrownell@xxxxxxxxxxxxxxxxxxxxx>
---
Noted around RC1(?) by someone who never sent the requested patch.
drivers/usb/gadget/u_serial.c | 16 +++++++---------
1 file changed, 7 insertions(+), 9 deletions(-)
--- a/drivers/usb/gadget/u_serial.c
+++ b/drivers/usb/gadget/u_serial.c
@@ -535,8 +535,9 @@ recycle:
list_move(&req->list, &port->read_pool);
}
- /* Push from tty to ldisc; this is immediate with low_latency, and
- * may trigger callbacks to this driver ... so drop the spinlock.
+ /* Push from tty to ldisc; this would be immediate with low_latency
+ * (no longer valid from tasklet context, though), and may trigger
+ * callbacks to this driver ... so drop the spinlock.
*/
if (tty && do_push) {
spin_unlock_irq(&port->port_lock);
@@ -555,14 +556,16 @@ recycle:
* implicit tty_unthrottle() call on its way...
*
* REVISIT we should probably add a timer to keep the tasklet
- * from starving ... but it's not clear that case ever happens.
+ * from starving: not throttled, but TTY layer is refusing data.
+ * This was observed on 2.6.27-rc1 when streaming megabytes of
+ * data down a raw pipe...
*/
if (!list_empty(queue) && tty) {
if (!test_bit(TTY_THROTTLED, &tty->flags)) {
if (do_push)
tasklet_schedule(&port->push);
else
- pr_warning(PREFIX "%d: RX not scheduled?\n",
+ pr_warning(PREFIX "%d: RX starving?\n",
port->port_num);
}
}
@@ -783,11 +786,6 @@ static int gs_open(struct tty_struct *tt
port->open_count = 1;
port->openclose = false;
- /* low_latency means ldiscs work in tasklet context, without
- * needing a workqueue schedule ... easier to keep up.
- */
- tty->low_latency = 1;
-
/* if connected, start the I/O stream */
if (port->port_usb) {
struct gserial *gser = port->port_usb;
--
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