Re: [PATCH] sound_core.c: Remove BKL from soundcore_open

[John Kacur <jkacur@redhat.com>]

On Sun, 11 Oct 2009, John Kacur wrote:

> 
> 
> On Sun, 11 Oct 2009, Frederic Weisbecker wrote:
> 
> > 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@redhat.com> wrote:
> > > > 
> > > > > >From 030af455d4f54482130c8eccb47fe90aaba8808c Mon Sep 17 00:00:00 2001
> > > > > From: John Kacur <jkacur@redhat.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@redhat.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@redhat.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....
> > 
> 
> Sigh... I do see your point - but on the otherhand if measurements don't
> show that mutex as being too coarse grained, then is it a problem?
> 
> Never-the-less here is version 3 of the patch - like Alan suggested, 
> punting, but at least reducing the area covered by the BKL.
> From ac9bdbdd192149e2498b6e16dc71f0a3933e1554 Mon Sep 17 00:00:00 2001
> From: John Kacur <jkacur@redhat.com>
> Date: Sun, 11 Oct 2009 14:25:46 +0200
> Subject: [PATCH] soundcore_open: Reduce the area BKL coverage in this function.
> 
> Most of this function is protected by the sound_loader_lock.
> We can push down the BKL to this call out err = file->f_op->open(inode,file);
> 
> Signed-off-by: John Kacur <jkacur@redhat.com>
> ---
>  sound/sound_core.c |    6 ++----
>  1 files changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/sound/sound_core.c b/sound/sound_core.c
> index 49c9981..a7d6956 100644
> --- a/sound/sound_core.c
> +++ b/sound/sound_core.c
> @@ -576,8 +576,6 @@ 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 +629,17 @@ 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)
> +			lock_kernel();
>  			err = file->f_op->open(inode,file);
> +			unlock_kernel();
>  		if (err) {
>  			fops_put(file->f_op);
>  			file->f_op = fops_get(old_fops);
>  		}
>  		fops_put(old_fops);
> -		unlock_kernel();
>  		return err;
>  	}
>  	spin_unlock(&sound_loader_lock);
> -	unlock_kernel();
>  	return -ENODEV;
>  }
>  
> -- 
> 1.6.0.6
> 

@Alan

Are you okay with this 3rd version of the patch that pushes the bkl lock
further down into the function so that it is only around the 
err = file->f_op->open(inode,file);

Not ideal - but an improvement and step in the right direction.

If so, maybe I can get an ack, so that Thomas might include it in his new
kill-the-bkl tree.

Thanks