Subject: Re: [PATCH tty-next] n_tty: Fix termios_rwsem lockdep false positive

From: Sergey Senozhatsky <[email-masked]>
Date: Mon, 12 Aug 2013 13:50:41 +0300

On (08/12/13 13:28), Artem Savkov wrote:
> Hi Peter,
> 
> On Sun, Aug 11, 2013 at 08:04:23AM -0400, Peter Hurley wrote:
> > Lockdep reports a circular lock dependency between
> > atomic_read_lock and termios_rwsem [1]. However, a lock
> > order deadlock is not possible since CPU1 only holds a
> > read lock which cannot prevent CPU0 from also acquiring
> > a read lock on the same r/w semaphore.
> > 
> > Unfortunately, lockdep cannot currently distinguish whether
> > the locks are read or write for any particular lock graph,
> > merely that the locks _were_ previously read and/or write.
> > 
> > Until lockdep is fixed, re-order atomic_read_lock so
> > termios_rwsem can be dropped and reacquired without
> > triggering lockdep.
> 
> Works fine, thanks.
> 
> Reported-and-tested-by: Artem Savkov <[email-masked]>
> 
> > Reported-by: Artem Savkov <[email-masked]>
> > Reported-by: Sergey Senozhatsky <[email-masked]>
> > Signed-off-by: Peter Hurley <[email-masked]>
> > 
> > [1] Initial lockdep report from Artem Savkov <[email-masked]>
> > 
> >  ======================================================
> >  [ INFO: possible circular locking dependency detected ]
> >  3.11.0-rc3-next-20130730+ #140 Tainted: G        W
> >  -------------------------------------------------------
> >  bash/1198 is trying to acquire lock:
> >   (&tty->termios_rwsem){++++..}, at: [<ffffffff816aa3bb>] n_tty_read+0x49b/0x660
> > 
> >  but task is already holding lock:
> >   (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff816aa0f0>] n_tty_read+0x1d0/0x660
> > 
> >  which lock already depends on the new lock.
> > 
> >  the existing dependency chain (in reverse order) is:
> > 
> >  -> #1 (&ldata->atomic_read_lock){+.+...}:
> >         [<ffffffff811111cc>] validate_chain+0x73c/0x850
> >         [<ffffffff811117e0>] __lock_acquire+0x500/0x5d0
> >         [<ffffffff81111a29>] lock_acquire+0x179/0x1d0
> >         [<ffffffff81d34b9c>] mutex_lock_interruptible_nested+0x7c/0x540
> >         [<ffffffff816aa0f0>] n_tty_read+0x1d0/0x660
> >         [<ffffffff816a3bb6>] tty_read+0x86/0xf0
> >         [<ffffffff811f21d3>] vfs_read+0xc3/0x130
> >         [<ffffffff811f2702>] SyS_read+0x62/0xa0
> >         [<ffffffff81d45259>] system_call_fastpath+0x16/0x1b
> > 
> >  -> #0 (&tty->termios_rwsem){++++..}:
> >         [<ffffffff8111064f>] check_prev_add+0x14f/0x590
> >         [<ffffffff811111cc>] validate_chain+0x73c/0x850
> >         [<ffffffff811117e0>] __lock_acquire+0x500/0x5d0
> >         [<ffffffff81111a29>] lock_acquire+0x179/0x1d0
> >         [<ffffffff81d372c1>] down_read+0x51/0xa0
> >         [<ffffffff816aa3bb>] n_tty_read+0x49b/0x660
> >         [<ffffffff816a3bb6>] tty_read+0x86/0xf0
> >         [<ffffffff811f21d3>] vfs_read+0xc3/0x130
> >         [<ffffffff811f2702>] SyS_read+0x62/0xa0
> >         [<ffffffff81d45259>] system_call_fastpath+0x16/0x1b
> > 
> >  other info that might help us debug this:
> > 
> >   Possible unsafe locking scenario:
> > 
> >         CPU0                    CPU1
> >         ----                    ----
> >    lock(&ldata->atomic_read_lock);
> >                                 lock(&tty->termios_rwsem);
> >                                 lock(&ldata->atomic_read_lock);
> >    lock(&tty->termios_rwsem);
> > 
> >   *** DEADLOCK ***
> > 
> >  2 locks held by bash/1198:
> >   #0:  (&tty->ldisc_sem){.+.+.+}, at: [<ffffffff816ade04>] tty_ldisc_ref_wait+0x24/0x60
> >   #1:  (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff816aa0f0>] n_tty_read+0x1d0/0x660
> > 
> >  stack backtrace:
> >  CPU: 1 PID: 1198 Comm: bash Tainted: G        W    3.11.0-rc3-next-20130730+ #140
> >  Hardware name: Bochs Bochs, BIOS Bochs 01/01/2007
> >   0000000000000000 ffff880019acdb28 ffffffff81d34074 0000000000000002
> >   0000000000000000 ffff880019acdb78 ffffffff8110ed75 ffff880019acdb98
> >   ffff880019fd0000 ffff880019acdb78 ffff880019fd0638 ffff880019fd0670
> >  Call Trace:
> >   [<ffffffff81d34074>] dump_stack+0x59/0x7d
> >   [<ffffffff8110ed75>] print_circular_bug+0x105/0x120
> >   [<ffffffff8111064f>] check_prev_add+0x14f/0x590
> >   [<ffffffff81d3ab5f>] ? _raw_spin_unlock_irq+0x4f/0x70
> >   [<ffffffff811111cc>] validate_chain+0x73c/0x850
> >   [<ffffffff8110ae0f>] ? trace_hardirqs_off_caller+0x1f/0x190
> >   [<ffffffff811117e0>] __lock_acquire+0x500/0x5d0
> >   [<ffffffff81111a29>] lock_acquire+0x179/0x1d0
> >   [<ffffffff816aa3bb>] ? n_tty_read+0x49b/0x660
> >   [<ffffffff81d372c1>] down_read+0x51/0xa0
> >   [<ffffffff816aa3bb>] ? n_tty_read+0x49b/0x660
> >   [<ffffffff816aa3bb>] n_tty_read+0x49b/0x660
> >   [<ffffffff810e4130>] ? try_to_wake_up+0x210/0x210
> >   [<ffffffff816a3bb6>] tty_read+0x86/0xf0
> >   [<ffffffff811f21d3>] vfs_read+0xc3/0x130
> >   [<ffffffff811f2702>] SyS_read+0x62/0xa0
> >   [<ffffffff815e24ee>] ? trace_hardirqs_on_thunk+0x3a/0x3f
> >   [<ffffffff81d45259>] system_call_fastpath+0x16/0x1b
> > ---
> >  drivers/tty/n_tty.c | 25 +++++++++++--------------
> >  1 file changed, 11 insertions(+), 14 deletions(-)
> >

I hate to do this, but isn't it actually my patch posted here
https://lkml.org/lkml/2013/8/1/510

which was tagged as `wrong'?

	-ss

> > diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
> > index dd8ae0c..c9a9ddd 100644
> > --- a/drivers/tty/n_tty.c
> > +++ b/drivers/tty/n_tty.c
> > @@ -2122,6 +2122,17 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
> >  	if (c < 0)
> >  		return c;
> >  
> > +	/*
> > +	 *	Internal serialization of reads.
> > +	 */
> > +	if (file->f_flags & O_NONBLOCK) {
> > +		if (!mutex_trylock(&ldata->atomic_read_lock))
> > +			return -EAGAIN;
> > +	} else {
> > +		if (mutex_lock_interruptible(&ldata->atomic_read_lock))
> > +			return -ERESTARTSYS;
> > +	}
> > +
> >  	down_read(&tty->termios_rwsem);
> >  
> >  	minimum = time = 0;
> > @@ -2141,20 +2152,6 @@ 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);
> > -			return -EAGAIN;
> > -		}
> > -	} else {
> > -		if (mutex_lock_interruptible(&ldata->atomic_read_lock)) {
> > -			up_read(&tty->termios_rwsem);
> > -			return -ERESTARTSYS;
> > -		}
> > -	}
> >  	packet = tty->packet;
> >  
> >  	add_wait_queue(&tty->read_wait, &wait);
> > -- 
> > 1.8.1.2
> > 
> 
> -- 
> Regards,
>     Artem
>