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tty: Remove tty_wait_until_sent_from_close()

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tty_wait_until_sent_from_close() drops the tty lock while waiting
for the tty driver to finish sending previously accepted data (ie.,
data remaining in its write buffer and transmit fifo).

tty_wait_until_sent_from_close() was added by commit a57a7bf3fc
("TTY: define tty_wait_until_sent_from_close") to prevent the entire
tty subsystem from being unable to open new ttys while waiting for
one tty to close while output drained.

However, since commit 0911261d4c ("tty: Don't take tty_mutex for tty
count changes"), holding a tty lock while closing does not prevent other
ttys from being opened/closed/hung up, but only prevents lifetime event
changes for the tty under lock.

Holding the tty lock while waiting for output to drain does prevent
parallel non-blocking opens (O_NONBLOCK) from advancing or returning
while the tty lock is held. However, all parallel opens _already_
block even if the tty lock is dropped while closing and the parallel
open advances. Blocking in open has been in mainline since at least 2.6.29
(see tty_port_block_til_ready(); note the test for O_NONBLOCK is _after_
the wait while ASYNC_CLOSING).

IOW, before this patch a non-blocking open will sleep anyway for the
_entire_ duration of a parallel hardware shutdown, and when it wakes, the
error return will cause a release of its tty, and it will restart with
a fresh attempt to open. Similarly with a blocking open that is already
waiting; when it's woken, the hardware shutdown has already completed
to ASYNC_INITIALIZED is not set, which forces a release and restart as
well.

So, holding the tty lock across the _entire_ close (which is what this
patch does), even while waiting for output to drain, is equivalent to
the current outcome wrt parallel opens.

Cc: Alan Cox <[email protected]>
Cc: David Laight <[email protected]>
CC: Arnd Bergmann <[email protected]>
CC: Karsten Keil <[email protected]>
CC: [email protected]
Signed-off-by: Peter Hurley <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
This commit is contained in:
Peter Hurley 2015-10-10 16:00:51 -04:00 • committed by Greg Kroah-Hartman
commit 79c1faa451
5 changed files with 5 additions and 30 deletions
Download patch file Download diff file Expand all files Collapse all files

2
drivers/tty/hvc/hvc_console.c
View file

@ -418,7 +418,7 @@ static void hvc_close(struct tty_struct *tty, struct file * filp)
* there is no buffered data otherwise sleeps on a wait queue
* waking periodically to check chars_in_buffer().
*/
tty_wait_until_sent_from_close(tty, HVC_CLOSE_WAIT);
tty_wait_until_sent(tty, HVC_CLOSE_WAIT);
} else {
if (hp->port.count < 0)
printk(KERN_ERR "hvc_close %X: oops, count is %d\n",

2
drivers/tty/hvc/hvcs.c
View file

@ -1230,7 +1230,7 @@ static void hvcs_close(struct tty_struct *tty, struct file *filp)
irq = hvcsd->vdev->irq;
spin_unlock_irqrestore(&hvcsd->lock, flags);
tty_wait_until_sent_from_close(tty, HVCS_CLOSE_WAIT);
tty_wait_until_sent(tty, HVCS_CLOSE_WAIT);
/*
* This line is important because it tells hvcs_open that this

11
drivers/tty/tty_port.c
View file

@ -463,10 +463,7 @@ static void tty_port_drain_delay(struct tty_port *port, struct tty_struct *tty)
schedule_timeout_interruptible(timeout);
}
/* Caller holds tty lock.
* NB: may drop and reacquire tty lock (in tty_wait_until_sent_from_close())
* so tty and tty port may have changed state (but not hung up or reopened).
*/
/* Caller holds tty lock. */
int tty_port_close_start(struct tty_port *port,
struct tty_struct *tty, struct file *filp)
{
@ -502,7 +499,7 @@ int tty_port_close_start(struct tty_port *port,
if (tty->flow_stopped)
tty_driver_flush_buffer(tty);
if (port->closing_wait != ASYNC_CLOSING_WAIT_NONE)
tty_wait_until_sent_from_close(tty, port->closing_wait);
tty_wait_until_sent(tty, port->closing_wait);
if (port->drain_delay)
tty_port_drain_delay(port, tty);
}
@ -543,10 +540,6 @@ EXPORT_SYMBOL(tty_port_close_end);
* tty_port_close
*
* Caller holds tty lock
*
* NB: may drop and reacquire tty lock (in tty_port_close_start()->
* tty_wait_until_sent_from_close()) so tty and tty_port may have changed
* state (but not hung up or reopened).
*/
void tty_port_close(struct tty_port *port, struct tty_struct *tty,
struct file *filp)