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Commit aa3cb814 authored by Peter Hurley's avatar Peter Hurley Committed by Greg Kroah-Hartman
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tty: Drop tty_mutex before tty reopen 



Holding tty_mutex for a tty re-open is no longer necessary since
"tty: Clarify re-open behavior of master ptys". Because the
slave tty count is no longer accessed by tty_reopen(), holding
tty_mutex to prevent concurrent final tty_release() of the slave
pty is not required.

As with "tty: Re-open /dev/tty without tty_mutex", holding a
tty kref until the tty_lock is acquired is sufficient to ensure
the tty has not been freed, which, in turn, is sufficient to
ensure the tty_lock can be safely acquired and the tty count
can be safely retrieved. A non-zero tty count with the tty lock
held guarantees that release_tty() has not run and cannot
run concurrently with tty_reopen().

Change tty_driver_lookup_tty() to acquire the tty kref, which
allows the tty_mutex to be dropped before acquiring the tty lock.
Dropping the tty_mutex before attempting the tty_lock allows
other ttys to be opened and released, without needing this
tty_reopen() to complete.

Reviewed-by: default avatarAlan Cox <alan@linux.intel.com>
Signed-off-by: default avatarPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 52494eeb

drivers/tty/tty_io.c

+17 −8
Original line number Diff line number Diff line
@@ -1344,19 +1344,24 @@ static ssize_t tty_line_name(struct tty_driver *driver, int index, char *p) 
 *	@driver: the driver for the tty

 *	@idx:	 the minor number

 *

 *	Return the tty, if found or ERR_PTR() otherwise.

 *	Return the tty, if found. If not found, return NULL or ERR_PTR() if the

 *	driver lookup() method returns an error.

 *

 *	Locking: tty_mutex must be held. If tty is found, the mutex must

 *	be held until the 'fast-open' is also done. Will change once we

 *	have refcounting in the driver and per driver locking

 *	Locking: tty_mutex must be held. If the tty is found, bump the tty kref.

 */

static struct tty_struct *tty_driver_lookup_tty(struct tty_driver *driver,

		struct inode *inode, int idx)

{

	struct tty_struct *tty;



	if (driver->ops->lookup)

		return driver->ops->lookup(driver, inode, idx);

		tty = driver->ops->lookup(driver, inode, idx);

	else

		tty = driver->ttys[idx];



	return driver->ttys[idx];

	if (!IS_ERR(tty))

		tty_kref_get(tty);

	return tty;

}



/**
@@ -2081,16 +2086,20 @@ static int tty_open(struct inode *inode, struct file *filp) 
		}



		if (tty) {

			mutex_unlock(&tty_mutex);

			tty_lock(tty);

			/* safe to drop the kref from tty_driver_lookup_tty() */

			tty_kref_put(tty);

			retval = tty_reopen(tty);

			if (retval < 0) {

				tty_unlock(tty);

				tty = ERR_PTR(retval);

			}

		} else	/* Returns with the tty_lock held for now */

		} else { /* Returns with the tty_lock held for now */

			tty = tty_init_dev(driver, index);



			mutex_unlock(&tty_mutex);

		}



		tty_driver_kref_put(driver);

	}