linux-kernel - Re: [PATCH] sound_core.c: Remove BKL from soundcore

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Date:	Sun, 11 Oct 2009 13:33:21 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	John Kacur <jkacur@...hat.com>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Jonathan Corbet <corbet@....net>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vincent^M^J Sanders <vince@...tec.co.uk>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] sound_core.c: Remove BKL from soundcore_open

On Sun, Oct 11, 2009 at 02:25:53AM +0200, John Kacur wrote:
> 
> 
> On Sun, 11 Oct 2009, Alan Cox wrote:
> 
> > On Sun, 11 Oct 2009 01:24:14 +0200 (CEST)
> > John Kacur <jkacur@...hat.com> wrote:
> > 
> > > >From 030af455d4f54482130c8eccb47fe90aaba8808c Mon Sep 17 00:00:00 2001
> > > From: John Kacur <jkacur@...hat.com>
> > > Date: Sat, 10 Oct 2009 23:39:56 +0200
> > > Subject: [PATCH] This code is already protected by spin_lock, and doesn't require the bkl
> > 
> > Sorry but I don't think that is true becaue of:
> > 
> >            spin_unlock(&sound_loader_lock);
> >                 if(file->f_op->open)
> >                         err = file->f_op->open(inode,file);
> > 
> > 
> > So the underlying driver open method expects lock_kernel status and you
> > don't propogate it down. You really need to track down each thing that
> > can be called into here and fix it, or maybe just punt for the moment and
> > push it down to
> > 
> > 	{
> > 		lock_kernel()
> > 		err = file-f_op->open ...
> > 		unlock_kernel()
> > 	}
> > 
> > so its obvious to the next person who takes up the war on the BKL what is
> > to be tackled.
> > 
> 
> Yikes, I missed that. Still I'm loath to just push it down like that. I 
> wonder if I can use a mutex there. What about the following patch?
> 
> From 8b0b91523ee2fcf60ccd82dba44b8da8bad34ce4 Mon Sep 17 00:00:00 2001
> From: John Kacur <jkacur@...hat.com>
> Date: Sun, 11 Oct 2009 02:14:44 +0200
> Subject: [PATCH] Remove the bkl in soundcore_open
> 
> Remove the bkl in soundcore_open since it is mostly covered by the sound_loader_lock spin_lock
> 
> Protect the underlying driver open method with a mutex.
> 
> Signed-off-by: John Kacur <jkacur@...hat.com>
> ---
>  sound/sound_core.c |    8 ++++----
>  1 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/sound/sound_core.c b/sound/sound_core.c
> index 49c9981..6afb6f1 100644
> --- a/sound/sound_core.c
> +++ b/sound/sound_core.c
> @@ -14,6 +14,8 @@
>  #include <linux/major.h>
>  #include <sound/core.h>
>  
> +static DEFINE_MUTEX(osc_mutex);
> +
>  #ifdef CONFIG_SOUND_OSS_CORE
>  static int __init init_oss_soundcore(void);
>  static void cleanup_oss_soundcore(void);
> @@ -576,8 +578,6 @@ static int soundcore_open(struct inode *inode, struct file *file)
>  	struct sound_unit *s;
>  	const struct file_operations *new_fops = NULL;
>  
> -	lock_kernel ();
> -
>  	chain=unit&0x0F;
>  	if(chain==4 || chain==5)	/* dsp/audio/dsp16 */
>  	{
> @@ -631,17 +631,17 @@ static int soundcore_open(struct inode *inode, struct file *file)
>  		file->f_op = new_fops;
>  		spin_unlock(&sound_loader_lock);
>  		if(file->f_op->open)
> +			mutex_lock(&osc_mutex);
>  			err = file->f_op->open(inode,file);
> +			mutex_unlock(&osc_mutex);


Yeah that's tempting, but I fear that also means this mutex will
never be removed....

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