Re: [PATCH] n_tty: release atomic_read_lock before calling schedule_timeout()

[Peter Hurley <peter@hurleysoftware.com>]

On 07/30/2013 11:35 AM, Artem Savkov wrote:
> ldata->atomic_read_lock should be released before scheduling as well as
> tty->termios_rwsem, otherwise there is a potential deadlock detected by lockdep

False positive.

> 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 ***

This situation is not possible since termios_rwsem is a read/write semaphore;
CPU1 cannot prevent CPU0 from obtaining a read lock on termios_rwsem.

This looks like a regression caused by:

commit a51805efae5dda0da66f79268ffcf0715f9dbea4
Author: Michel Lespinasse <walken@google.com>
Date:   Mon Jul 8 14:23:49 2013 -0700

     lockdep: Introduce lock_acquire_exclusive()/shared() helper macros

     In lockdep.h, the spinlock/mutex/rwsem/rwlock/lock_map acquire macros have
     different definitions based on the value of CONFIG_PROVE_LOCKING.  We have
     separate ifdefs for each of these definitions, which seems redundant.

     Introduce lock_acquire_{exclusive,shared,shared_recursive} helpers which
     will have different definitions based on CONFIG_PROVE_LOCKING.  Then all
     other helper macros can be defined based on the above ones, which reduces
     the amount of ifdefined code.

     Signed-off-by: Michel Lespinasse <walken@google.com>
     Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
     Signed-off-by: Peter Zijlstra <peterz@infradead.org>
     Cc: Oleg Nesterov <oleg@redhat.com>
     Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
     Cc: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
     Cc: Rusty Russell <rusty@rustcorp.com.au>
     Cc: Andi Kleen <ak@linux.intel.com>
     Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
     Cc: Steven Rostedt <rostedt@goodmis.org>
     Link: http://lkml.kernel.org/r/20130708212350.6DD1931C15E@corp2gmr1-1.hot.corp.google.com
     Signed-off-by: Ingo Molnar <mingo@kernel.org>


> [   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);
>