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

[Artem Savkov]

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 <artem.savkov@xxxxxxxxx>

> Reported-by: Artem Savkov <artem.savkov@xxxxxxxxx>
> Reported-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
> Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
> 
> [1] Initial lockdep report from Artem Savkov <artem.savkov@xxxxxxxxx>
> 
>  ======================================================
>  [ 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(-)
> 
> 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
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