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Subject[PATCH] n_tty: release atomic_read_lock before calling schedule_timeout()
Date
ldata->atomic_read_lock should be released before scheduling as well as
tty->termios_rwsem, otherwise there is a potential deadlock detected by lockdep

Introduced in "n_tty: Access termios values safely"
(9356b535fcb71db494fc434acceb79f56d15bda2 in linux-next.git)

[ 16.822058] ======================================================
[ 16.822058] [ INFO: possible circular locking dependency detected ]
[ 16.822058] 3.11.0-rc3-next-20130730+ #140 Tainted: G W
[ 16.822058] -------------------------------------------------------
[ 16.822058] bash/1198 is trying to acquire lock:
[ 16.822058] (&tty->termios_rwsem){++++..}, at: [<ffffffff816aa3bb>] n_tty_read+0x49b/0x660
[ 16.822058]
[ 16.822058] but task is already holding lock:
[ 16.822058] (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff816aa0f0>] n_tty_read+0x1d0/0x660
[ 16.822058]
[ 16.822058] which lock already depends on the new lock.
[ 16.822058]
[ 16.822058]
[ 16.822058] the existing dependency chain (in reverse order) is:
[ 16.822058]
-> #1 (&ldata->atomic_read_lock){+.+...}:
[ 16.822058] [<ffffffff811111cc>] validate_chain+0x73c/0x850
[ 16.822058] [<ffffffff811117e0>] __lock_acquire+0x500/0x5d0
[ 16.822058] [<ffffffff81111a29>] lock_acquire+0x179/0x1d0
[ 16.822058] [<ffffffff81d34b9c>] mutex_lock_interruptible_nested+0x7c/0x540
[ 16.822058] [<ffffffff816aa0f0>] n_tty_read+0x1d0/0x660
[ 16.822058] [<ffffffff816a3bb6>] tty_read+0x86/0xf0
[ 16.822058] [<ffffffff811f21d3>] vfs_read+0xc3/0x130
[ 16.822058] [<ffffffff811f2702>] SyS_read+0x62/0xa0
[ 16.822058] [<ffffffff81d45259>] system_call_fastpath+0x16/0x1b
[ 16.822058]
-> #0 (&tty->termios_rwsem){++++..}:
[ 16.822058] [<ffffffff8111064f>] check_prev_add+0x14f/0x590
[ 16.822058] [<ffffffff811111cc>] validate_chain+0x73c/0x850
[ 16.822058] [<ffffffff811117e0>] __lock_acquire+0x500/0x5d0
[ 16.822058] [<ffffffff81111a29>] lock_acquire+0x179/0x1d0
[ 16.822058] [<ffffffff81d372c1>] down_read+0x51/0xa0
[ 16.822058] [<ffffffff816aa3bb>] n_tty_read+0x49b/0x660
[ 16.822058] [<ffffffff816a3bb6>] tty_read+0x86/0xf0
[ 16.822058] [<ffffffff811f21d3>] vfs_read+0xc3/0x130
[ 16.822058] [<ffffffff811f2702>] SyS_read+0x62/0xa0
[ 16.822058] [<ffffffff81d45259>] system_call_fastpath+0x16/0x1b
[ 16.822058]
[ 16.822058] other info that might help us debug this:
[ 16.822058]
[ 16.822058] Possible unsafe locking scenario:
[ 16.822058]
[ 16.822058] CPU0 CPU1
[ 16.822058] ---- ----
[ 16.822058] lock(&ldata->atomic_read_lock);
[ 16.822058] lock(&tty->termios_rwsem);
[ 16.822058] lock(&ldata->atomic_read_lock);
[ 16.822058] lock(&tty->termios_rwsem);
[ 16.822058]
[ 16.822058] *** DEADLOCK ***
[ 16.822058]
[ 16.822058] 2 locks held by bash/1198:
[ 16.822058] #0: (&tty->ldisc_sem){.+.+.+}, at: [<ffffffff816ade04>] tty_ldisc_ref_wait+0x24/0x60
[ 16.822058] #1: (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff816aa0f0>] n_tty_read+0x1d0/0x660
[ 16.822058]
[ 16.822058] stack backtrace:
[ 16.822058] CPU: 1 PID: 1198 Comm: bash Tainted: G W 3.11.0-rc3-next-20130730+ #140
[ 16.822058] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2007
[ 16.822058] 0000000000000000 ffff880019acdb28 ffffffff81d34074 0000000000000002
[ 16.822058] 0000000000000000 ffff880019acdb78 ffffffff8110ed75 ffff880019acdb98
[ 16.822058] ffff880019fd0000 ffff880019acdb78 ffff880019fd0638 ffff880019fd0670
[ 16.822058] Call Trace:
[ 16.822058] [<ffffffff81d34074>] dump_stack+0x59/0x7d
[ 16.822058] [<ffffffff8110ed75>] print_circular_bug+0x105/0x120
[ 16.822058] [<ffffffff8111064f>] check_prev_add+0x14f/0x590
[ 16.822058] [<ffffffff81d3ab5f>] ? _raw_spin_unlock_irq+0x4f/0x70
[ 16.822058] [<ffffffff811111cc>] validate_chain+0x73c/0x850
[ 16.822058] [<ffffffff8110ae0f>] ? trace_hardirqs_off_caller+0x1f/0x190
[ 16.822058] [<ffffffff811117e0>] __lock_acquire+0x500/0x5d0
[ 16.822058] [<ffffffff81111a29>] lock_acquire+0x179/0x1d0
[ 16.822058] [<ffffffff816aa3bb>] ? n_tty_read+0x49b/0x660
[ 16.822058] [<ffffffff81d372c1>] down_read+0x51/0xa0
[ 16.822058] [<ffffffff816aa3bb>] ? n_tty_read+0x49b/0x660
[ 16.822058] [<ffffffff816aa3bb>] n_tty_read+0x49b/0x660
[ 16.822058] [<ffffffff810e4130>] ? try_to_wake_up+0x210/0x210
[ 16.822058] [<ffffffff816a3bb6>] tty_read+0x86/0xf0
[ 16.822058] [<ffffffff811f21d3>] vfs_read+0xc3/0x130
[ 16.822058] [<ffffffff811f2702>] SyS_read+0x62/0xa0
[ 16.822058] [<ffffffff815e24ee>] ? trace_hardirqs_on_thunk+0x3a/0x3f
[ 16.822058] [<ffffffff81d45259>] system_call_fastpath+0x16/0x1b

Signed-off-by: Artem Savkov <artem.savkov@gmail.com>
---
drivers/tty/n_tty.c | 12 ++++++++++++
1 files changed, 12 insertions(+), 0 deletions(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index dd8ae0c..38c09db 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -2203,11 +2203,23 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
break;
}
n_tty_set_room(tty);
+ mutex_unlock(&ldata->atomic_read_lock);
up_read(&tty->termios_rwsem);

timeout = schedule_timeout(timeout);

down_read(&tty->termios_rwsem);
+ if (file->f_flags & O_NONBLOCK) {
+ if (!mutex_trylock(&ldata->atomic_read_lock)) {
+ retval = -EAGAIN;
+ break;
+ }
+ } else {
+ if (mutex_lock_interruptible(&ldata->atomic_read_lock)) {
+ retval = -ERESTARTSYS;
+ break;
+ }
+ }
continue;
}
__set_current_state(TASK_RUNNING);
--
1.7.0.4


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