[PATCH -next v2 07/26] tty: Re-open /dev/tty without tty_mutex

Peter Hurley
2014-11-05T17:21:37Z

Opening /dev/tty (ie., the controlling tty for the current task)
is always a re-open of the underlying tty. Because holding the
tty_lock is sufficient for safely re-opening a tty, and because
having a tty kref is sufficient for safely acquiring the tty_lock [1],
tty_open_current_tty() does not require holding tty_mutex.

Repurpose tty_open_current_tty() to perform the re-open itself and
refactor tty_open().

[1] Analysis of safely re-opening the current tty w/o tty_mutex

get_current_tty() gets a tty kref from the already kref'ed tty value of
current->signal->tty while holding the sighand lock for the current
task. This guarantees that the tty pointer returned from
get_current_tty() points to a tty which remains referenceable
while holding the kref.

Although release_tty() may run concurrently, and thus the driver
reference may be removed, release_one_tty() cannot have run, and
won't while holding the tty kref.

This, in turn, guarantees the tty_lock() can safely be acquired
(since tty->magic and tty->legacy_mutex are still a valid dereferences).
The tty_lock() also gets a tty kref to prevent the tty_unlock() from
dereferencing a released tty. Thus, the kref returned from
get_current_tty() can be released.

Lastly, the first operation of tty_reopen() is to check the tty count.
If non-zero, this ensures release_tty() is not running concurrently,
and the driver references have not been removed.

Reviewed-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
---
drivers/tty/tty_io.c | 53 ++++++++++++++++++++++++++--------------------------
1 file changed, 27 insertions(+), 26 deletions(-)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 97445c2..fb4583c 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1944,20 +1944,20 @@ int tty_release(struct inode *inode, struct file *filp)
}

/**
- * tty_open_current_tty - get tty of current task for open
+ * tty_open_current_tty - get locked tty of current task
* @device: device number
* @filp: file pointer to tty
- * @return: tty of the current task iff @device is /dev/tty
+ * @return: locked tty of the current task iff @device is /dev/tty
+ *
+ * Performs a re-open of the current task's controlling tty.
*
* We cannot return driver and index like for the other nodes because
* devpts will not work then. It expects inodes to be from devpts FS.
- *
- * We need to move to returning a refcounted object from all the lookup
- * paths including this one.
*/
static struct tty_struct *tty_open_current_tty(dev_t device, struct file *filp)
{
struct tty_struct *tty;
+ int retval;

if (device != MKDEV(TTYAUX_MAJOR, 0))
return NULL;
@@ -1968,9 +1968,14 @@ static struct tty_struct *tty_open_current_tty(dev_t device, struct file *filp)

filp->f_flags |= O_NONBLOCK; /* Don't let /dev/tty block */
/* noctty = 1; */
- tty_kref_put(tty);
- /* FIXME: we put a reference and return a TTY! */
- /* This is only safe because the caller holds tty_mutex */
+ tty_lock(tty);
+ tty_kref_put(tty); /* safe to drop the kref now */
+
+ retval = tty_reopen(tty);
+ if (retval < 0) {
+ tty_unlock(tty);
+ tty = ERR_PTR(retval);
+ }
return tty;
}

@@ -2068,13 +2073,9 @@ retry_open:
index = -1;
retval = 0;

- mutex_lock(&tty_mutex);
- /* This is protected by the tty_mutex */
tty = tty_open_current_tty(device, filp);
- if (IS_ERR(tty)) {
- retval = PTR_ERR(tty);
- goto err_unlock;
- } else if (!tty) {
+ if (!tty) {
+ mutex_lock(&tty_mutex);
driver = tty_lookup_driver(device, filp, &noctty, &index);
if (IS_ERR(driver)) {
retval = PTR_ERR(driver);
@@ -2087,21 +2088,21 @@ retry_open:
retval = PTR_ERR(tty);
goto err_unlock;
}
- }

- if (tty) {
- tty_lock(tty);
- retval = tty_reopen(tty);
- if (retval < 0) {
- tty_unlock(tty);
- tty = ERR_PTR(retval);
- }
- } else /* Returns with the tty_lock held for now */
- tty = tty_init_dev(driver, index);
+ if (tty) {
+ tty_lock(tty);
+ retval = tty_reopen(tty);
+ if (retval < 0) {
+ tty_unlock(tty);
+ tty = ERR_PTR(retval);
+ }
+ } else /* Returns with the tty_lock held for now */
+ tty = tty_init_dev(driver, index);

- mutex_unlock(&tty_mutex);
- if (driver)
+ mutex_unlock(&tty_mutex);
tty_driver_kref_put(driver);
+ }
+
if (IS_ERR(tty)) {
retval = PTR_ERR(tty);
goto err_file;
--
2.1.3