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authorPeter Hurley <peter@hurleysoftware.com>2014-02-22 07:31:21 -0500
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2014-04-26 17:19:04 -0700
commit4f1f4df2c1aee858da70f91970f6c9cb651a63de (patch)
parent192ae8160c9465dd3f0c6650b29cc4ad019bc4e2 (diff)
tty: Fix low_latency BUG
commit a9c3f68f3cd8d55f809fbdb0c138ed061ea1bd25 upstream. The user-settable knob, low_latency, has been the source of several BUG reports which stem from flush_to_ldisc() running in interrupt context. Since 3.12, which added several sleeping locks (termios_rwsem and buf->lock) to the input processing path, the frequency of these BUG reports has increased. Note that changes in 3.12 did not introduce this regression; sleeping locks were first added to the input processing path with the removal of the BKL from N_TTY in commit a88a69c91256418c5907c2f1f8a0ec0a36f9e6cc, 'n_tty: Fix loss of echoed characters and remove bkl from n_tty' and later in commit 38db89799bdf11625a831c5af33938dcb11908b6, 'tty: throttling race fix'. Since those changes, executing flush_to_ldisc() in interrupt_context (ie, low_latency set), is unsafe. However, since most devices do not validate if the low_latency setting is appropriate for the context (process or interrupt) in which they receive data, some reports are due to misconfiguration. Further, serial dma devices for which dma fails, resort to interrupt receiving as a backup without resetting low_latency. Historically, low_latency was used to force wake-up the reading process rather than wait for the next scheduler tick. The effect was to trim multiple milliseconds of latency from when the process would receive new data. Recent tests [1] have shown that the reading process now receives data with only 10's of microseconds latency without low_latency set. Remove the low_latency rx steering from tty_flip_buffer_push(); however, leave the knob as an optional hint to drivers that can tune their rx fifos and such like. Cleanup stale code comments regarding low_latency. [1] https://lkml.org/lkml/2014/2/20/434 "Yay.. thats an annoying historical pain in the butt gone." -- Alan Cox Reported-by: Beat Bolli <bbolli@ewanet.ch> Reported-by: Pavel Roskin <proski@gnu.org> Acked-by: David Sterba <dsterba@suse.cz> Cc: Grant Edwards <grant.b.edwards@gmail.com> Cc: Stanislaw Gruszka <sgruszka@redhat.com> Cc: Hal Murray <murray+fedora@ip-64-139-1-69.sjc.megapath.net> Signed-off-by: Peter Hurley <peter@hurleysoftware.com> Signed-off-by: Alan Cox <alan@linux.intel.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
4 files changed, 7 insertions, 22 deletions
diff --git a/drivers/tty/ipwireless/tty.c b/drivers/tty/ipwireless/tty.c
index ebd5bff0f5c1..17ee3bf0926b 100644
--- a/drivers/tty/ipwireless/tty.c
+++ b/drivers/tty/ipwireless/tty.c
@@ -176,9 +176,6 @@ void ipwireless_tty_received(struct ipw_tty *tty, unsigned char *data,
": %d chars not inserted to flip buffer!\n",
length - work);
- /*
- * This may sleep if ->low_latency is set
- */
if (work)
diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 765125dff20e..8ebd9f88a6f6 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -351,14 +351,11 @@ EXPORT_SYMBOL(tty_insert_flip_string_flags);
* Takes any pending buffers and transfers their ownership to the
* ldisc side of the queue. It then schedules those characters for
* processing by the line discipline.
- * Note that this function can only be used when the low_latency flag
- * is unset. Otherwise the workqueue won't be flushed.
void tty_schedule_flip(struct tty_port *port)
struct tty_bufhead *buf = &port->buf;
- WARN_ON(port->low_latency);
buf->tail->commit = buf->tail->used;
@@ -482,17 +479,15 @@ static void flush_to_ldisc(struct work_struct *work)
void tty_flush_to_ldisc(struct tty_struct *tty)
- if (!tty->port->low_latency)
- flush_work(&tty->port->buf.work);
+ flush_work(&tty->port->buf.work);
* tty_flip_buffer_push - terminal
* @port: tty port to push
- * Queue a push of the terminal flip buffers to the line discipline. This
- * function must not be called from IRQ context if port->low_latency is
- * set.
+ * Queue a push of the terminal flip buffers to the line discipline.
+ * Can be called from IRQ/atomic context.
* In the event of the queue being busy for flipping the work will be
* held off and retried later.
@@ -500,14 +495,7 @@ void tty_flush_to_ldisc(struct tty_struct *tty)
void tty_flip_buffer_push(struct tty_port *port)
- struct tty_bufhead *buf = &port->buf;
- buf->tail->commit = buf->tail->used;
- if (port->low_latency)
- flush_to_ldisc(&buf->work);
- else
- schedule_work(&buf->work);
+ tty_schedule_flip(port);
diff --git a/drivers/usb/gadget/u_serial.c b/drivers/usb/gadget/u_serial.c
index b369292d4b90..ad0aca812002 100644
--- a/drivers/usb/gadget/u_serial.c
+++ b/drivers/usb/gadget/u_serial.c
@@ -549,8 +549,8 @@ static void gs_rx_push(unsigned long _port)
- /* Push from tty to ldisc; without low_latency set this is handled by
- * a workqueue, so we won't get callbacks and can hold port_lock
+ /* Push from tty to ldisc; this is handled by a workqueue,
+ * so we won't get callbacks and can hold port_lock
if (do_push)
diff --git a/include/linux/tty.h b/include/linux/tty.h
index 90b4fdc8a61f..b90b5c221ff0 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -208,7 +208,7 @@ struct tty_port {
wait_queue_head_t delta_msr_wait; /* Modem status change */
unsigned long flags; /* TTY flags ASY_*/
unsigned char console:1, /* port is a console */
- low_latency:1; /* direct buffer flush */
+ low_latency:1; /* optional: tune for latency */
struct mutex mutex; /* Locking */
struct mutex buf_mutex; /* Buffer alloc lock */
unsigned char *xmit_buf; /* Optional buffer */