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Subject[PATCH next] n_tty: change lock ordering in n_tty_read() (v2)
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

v2: correct subject typo.

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;
}



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