[PATCH next] n_tty: change lock oreding in n_tty_read()

[Sergey Senozhatsky <sergey.senozhatsky@gmail.com>]

Acquire atomic_read_lock before termios_rwsem rw-lock in n_tty_read().
termios_rwsem is getting released and re-acquired, while remaining
readers are blocked on atomic_read_lock mutex, in case when process
must wait for input_available_p(), making lockdep unhappy:

[  463.542463] ======================================================
[  463.542464] [ INFO: possible circular locking dependency detected ]
[  463.542466] 3.11.0-rc3-next-20130801-dbg-dirty #5 Not tainted
[  463.542467] -------------------------------------------------------
[  463.542469] agetty/2075 is trying to acquire lock:
[  463.542484]  (&tty->termios_rwsem){++++..}, at: [<ffffffff81398660>] n_tty_read+0x460/0xab0
[  463.542484] 
but task is already holding lock:
[  463.542496]  (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff81398959>] n_tty_read+0x759/0xab0
[  463.542497] 
which lock already depends on the new lock.

[  463.542497] 
the existing dependency chain (in reverse order) is:
[  463.542502] 
-> #1 (&ldata->atomic_read_lock){+.+...}:
[  463.542508]        [<ffffffff810b9394>] lock_acquire+0xa4/0x200
[  463.542514]        [<ffffffff8160a787>] mutex_lock_interruptible_nested+0x77/0x4e0
[  463.542518]        [<ffffffff81398959>] n_tty_read+0x759/0xab0
[  463.542524]        [<ffffffff81393e8d>] tty_read+0x8d/0x100
[  463.542528]        [<ffffffff8117987a>] vfs_read+0x9a/0x170
[  463.542531]        [<ffffffff8117a05c>] SyS_read+0x4c/0xa0
[  463.542536]        [<ffffffff81610c7e>] tracesys+0xd0/0xd5
[  463.542540] 
-> #0 (&tty->termios_rwsem){++++..}:
[  463.542543]        [<ffffffff810b84e6>] __lock_acquire+0x1756/0x1d20
[  463.542547]        [<ffffffff810b9394>] lock_acquire+0xa4/0x200
[  463.542550]        [<ffffffff8160c1b7>] down_read+0x47/0x60
[  463.542554]        [<ffffffff81398660>] n_tty_read+0x460/0xab0
[  463.542557]        [<ffffffff81393e8d>] tty_read+0x8d/0x100
[  463.542560]        [<ffffffff8117987a>] vfs_read+0x9a/0x170
[  463.542563]        [<ffffffff8117a05c>] SyS_read+0x4c/0xa0
[  463.542566]        [<ffffffff81610c7e>] tracesys+0xd0/0xd5
[  463.542567] 
other info that might help us debug this:

[  463.542569]  Possible unsafe locking scenario:

[  463.542570]        CPU0                    CPU1
[  463.542570]        ----                    ----
[  463.542573]   lock(&ldata->atomic_read_lock);
[  463.542575]                                lock(&tty->termios_rwsem);
[  463.542577]                                lock(&ldata->atomic_read_lock);
[  463.542579]   lock(&tty->termios_rwsem);
[  463.542580] 
 *** DEADLOCK ***

[  463.542582] 2 locks held by agetty/2075:
[  463.542590]  #0:  (&tty->ldisc_sem){++++++}, at: [<ffffffff8139c130>] tty_ldisc_ref_wait+0x20/0x50
[  463.542597]  #1:  (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff81398959>] n_tty_read+0x759/0xab0
[  463.542598] 
stack backtrace:
[  463.542602] CPU: 0 PID: 2075 Comm: agetty Not tainted 3.11.0-rc3-next-20130801-dbg-dirty #5
[  463.542603] Hardware name: Acer             Aspire 5741G    /Aspire 5741G    , BIOS V1.20 02/08/2011
[  463.542609]  ffffffff82236010 ffff88009af67c28 ffffffff8160630a ffffffff82236010
[  463.542613]  ffff88009af67c68 ffffffff816031b7 ffff880151ac0000 ffff880151ac0790
[  463.542617]  000000073469e1bd ffff880151ac0768 ffff880151ac0790 ffff880151ac0000
[  463.542618] Call Trace:
[  463.542625]  [<ffffffff8160630a>] dump_stack+0x4e/0x82
[  463.542630]  [<ffffffff816031b7>] print_circular_bug+0x2b6/0x2c5
[  463.542634]  [<ffffffff810b84e6>] __lock_acquire+0x1756/0x1d20
[  463.542638]  [<ffffffff810b9394>] lock_acquire+0xa4/0x200
[  463.542642]  [<ffffffff81398660>] ? n_tty_read+0x460/0xab0
[  463.542645]  [<ffffffff8160c1b7>] down_read+0x47/0x60
[  463.542649]  [<ffffffff81398660>] ? n_tty_read+0x460/0xab0
[  463.542653]  [<ffffffff81398660>] n_tty_read+0x460/0xab0
[  463.542661]  [<ffffffff81085020>] ? wake_up_process+0x40/0x40
[  463.542665]  [<ffffffff81393e8d>] tty_read+0x8d/0x100
[  463.542668]  [<ffffffff8117987a>] vfs_read+0x9a/0x170
[  463.542671]  [<ffffffff8117a05c>] SyS_read+0x4c/0xa0
[  463.542674]  [<ffffffff81610c7e>] tracesys+0xd0/0xd5

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>

---

 drivers/tty/n_tty.c | 15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index dd8ae0c..03bd6e8 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -2122,8 +2122,6 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
 	if (c < 0)
 		return c;
 
-	down_read(&tty->termios_rwsem);
-
 	minimum = time = 0;
 	timeout = MAX_SCHEDULE_TIMEOUT;
 	if (!ldata->icanon) {
@@ -2145,16 +2143,15 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
 	 *	Internal serialization of reads.
 	 */
 	if (file->f_flags & O_NONBLOCK) {
-		if (!mutex_trylock(&ldata->atomic_read_lock)) {
-			up_read(&tty->termios_rwsem);
+		if (!mutex_trylock(&ldata->atomic_read_lock))
 			return -EAGAIN;
-		}
 	} else {
-		if (mutex_lock_interruptible(&ldata->atomic_read_lock)) {
-			up_read(&tty->termios_rwsem);
+		if (mutex_lock_interruptible(&ldata->atomic_read_lock))
 			return -ERESTARTSYS;
-		}
 	}
+
+	down_read(&tty->termios_rwsem);
+
 	packet = tty->packet;
 
 	add_wait_queue(&tty->read_wait, &wait);
@@ -2248,7 +2245,6 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
 		if (time)
 			timeout = time;
 	}
-	mutex_unlock(&ldata->atomic_read_lock);
 	remove_wait_queue(&tty->read_wait, &wait);
 
 	if (!waitqueue_active(&tty->read_wait))
@@ -2260,6 +2256,7 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
 
 	n_tty_set_room(tty);
 	up_read(&tty->termios_rwsem);
+	mutex_unlock(&ldata->atomic_read_lock);
 	return retval;
 }