tty: Drop tty_mutex before tty reopen (aa3cb814) · Commits · 戴 / test

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>

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 @@ 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);
		}

		tty_driver_kref_put(driver);
		}

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