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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: Alan Cox <alan@linux.intel.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Peter Hurley 2014-11-05 12:12:51 -05:00 committed by Greg Kroah-Hartman
parent 52494eeb99
commit aa3cb814a8
1 changed files with 18 additions and 9 deletions
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27
drivers/tty/tty_io.c
View File

@ -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)
{
if (driver->ops->lookup)
return driver->ops->lookup(driver, inode, idx);
struct tty_struct *tty;
return driver->ttys[idx];
if (driver->ops->lookup)
tty = driver->ops->lookup(driver, inode, idx);
else
tty = 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);
}
mutex_unlock(&tty_mutex);
tty_driver_kref_put(driver);
}