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>
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@ -1344,19 +1344,24 @@ static ssize_t tty_line_name(struct tty_driver *driver, int index, char *p)
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* @driver: the driver for the tty
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* @idx: the minor number
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*
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* Return the tty, if found or ERR_PTR() otherwise.
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* Return the tty, if found. If not found, return NULL or ERR_PTR() if the
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* driver lookup() method returns an error.
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*
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* Locking: tty_mutex must be held. If tty is found, the mutex must
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* be held until the 'fast-open' is also done. Will change once we
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* have refcounting in the driver and per driver locking
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* Locking: tty_mutex must be held. If the tty is found, bump the tty kref.
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*/
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static struct tty_struct *tty_driver_lookup_tty(struct tty_driver *driver,
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struct inode *inode, int idx)
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{
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if (driver->ops->lookup)
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return driver->ops->lookup(driver, inode, idx);
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struct tty_struct *tty;
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return driver->ttys[idx];
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if (driver->ops->lookup)
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tty = driver->ops->lookup(driver, inode, idx);
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else
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tty = driver->ttys[idx];
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if (!IS_ERR(tty))
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tty_kref_get(tty);
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return tty;
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}
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/**
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@ -2081,16 +2086,20 @@ static int tty_open(struct inode *inode, struct file *filp)
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}
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if (tty) {
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mutex_unlock(&tty_mutex);
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tty_lock(tty);
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/* safe to drop the kref from tty_driver_lookup_tty() */
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tty_kref_put(tty);
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retval = tty_reopen(tty);
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if (retval < 0) {
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tty_unlock(tty);
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tty = ERR_PTR(retval);
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}
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} else /* Returns with the tty_lock held for now */
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} else { /* Returns with the tty_lock held for now */
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tty = tty_init_dev(driver, index);
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mutex_unlock(&tty_mutex);
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}
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mutex_unlock(&tty_mutex);
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tty_driver_kref_put(driver);
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}
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