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.senozhat...@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; } -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/