[PATCH -next 08/27] tty: Drop tty_mutex before tty reopen

Peter Hurley
2014-10-16T20:26:31Z

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.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
---
drivers/tty/tty_io.c | 25 +++++++++++++++++--------
1 file changed, 17 insertions(+), 8 deletions(-)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 1b6f38c..ded0445 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -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;
}

/**
@@ -2089,16 +2094,20 @@ retry_open:
}

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);
+ }

- mutex_unlock(&tty_mutex);
tty_driver_kref_put(driver);
}

--
2.1.1