linux-kernel - Re: [PATCH] tty: tty_mutex: fix lockdep warning in tty_lock

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Date:	Sat, 26 May 2012 09:16:04 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, Ming Lei <tom.leiming@...il.com>,
	Alan Cox <alan@...ux.intel.com>, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] tty: tty_mutex: fix lockdep warning in tty_lock_pair(v3)

On Sat, 2012-05-26 at 10:54 +0800, Ming Lei wrote:
> From: Ming Lei <tom.leiming@...il.com>

> Cc: Alan Cox <alan@...ux.intel.com>
> Cc: Arnd Bergmann <arnd@...db.de>
> Signed-off-by: Peter Zijlstra <peterz@...radead.org>

Oh very much not!

> Signed-off-by: Ming Lei <ming.lei@...onical.com>
> ---
> v3:
> 	fix unlock order in tty_unlock_pair
> 
>  drivers/tty/tty_mutex.c |   28 +++++++++++++++++++++-------
>  1 file changed, 21 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/tty/tty_mutex.c b/drivers/tty/tty_mutex.c
> index 69adc80..c7f4523 100644

> @@ -43,11 +49,14 @@ void __lockfunc tty_lock_pair(struct tty_struct *tty,
>  {
>  	if (tty < tty2) {
>  		tty_lock(tty);
> -		tty_lock(tty2);
> +		tty_lock_nested(tty2, SINGLE_DEPTH_NESTING);
>  	} else {
> -		if (tty2 && tty2 != tty)
> +		int nested = 0;
> +		if (tty2 && tty2 != tty) {
>  			tty_lock(tty2);
> -		tty_lock(tty);
> +			nested = SINGLE_DEPTH_NESTING;
> +		}
> +		tty_lock_nested(tty, nested);
>  	}
>  }
>  EXPORT_SYMBOL(tty_lock_pair);


I've still to hear what's wrong with a simple:


  if (!tty2 || tty == tty2) {
	tty_lock(tty);
	return;
  }

  if (tty > tty2)
	swap(tty, tty2);

  tty_lock(tty);
  tty_lock_nested(tty2, SINGLE_DEPTH_NESTING);


That's a lot more readable than the proposed code.

> @@ -55,8 +64,13 @@ EXPORT_SYMBOL(tty_lock_pair);
>  void __lockfunc tty_unlock_pair(struct tty_struct *tty,
>  						struct tty_struct *tty2)
>  {
> -	tty_unlock(tty);
> -	if (tty2 && tty2 != tty)
> +	if (tty < tty2) {
>  		tty_unlock(tty2);
> +		tty_unlock(tty);
> +	} else {
> +		tty_unlock(tty);
> +		if (tty2 && tty2 != tty)
> +			tty_unlock(tty2);
> +	}
>  }
>  EXPORT_SYMBOL(tty_unlock_pair);

This is complete crap, unlock order doesn't matter.

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