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


    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



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