linux-kernel - Re: [PATCH] input: remove BKL from uinput open function

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Message-ID: <20100201212132.GA7380@core.coreip.homeip.net>
Date:	Mon, 1 Feb 2010 13:21:33 -0800
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	John Kacur <jkacur@...il.com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Thadeu Lima de Souza Cascardo <cascardo@...oscopio.com>,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] input: remove BKL from uinput open function

On Mon, Feb 01, 2010 at 09:27:22PM +0100, John Kacur wrote:
> On Mon, Feb 1, 2010 at 9:22 PM, John Kacur <jkacur@...hat.com> wrote:
> > On Sun, Jan 31, 2010 at 6:29 AM, Dmitry Torokhov
> > <dmitry.torokhov@...il.com> wrote:
> >> On Sun, Jan 31, 2010 at 05:20:55AM +0100, Arnd Bergmann wrote:
> >>> On Sunday 31 January 2010, John Kacur wrote:
> >>> > > Sorry, I should have been clearer, but not implementing llseek
> >>> > > is the problem I was referring to: When a driver has no explicit
> >>> > > .llseek operation in its file operations and does not call
> >>> > > nonseekable_open from its open operation, the VFS layer will
> >>> > > implicitly use default_llseek, which takes the BKL. We're
> >>> > > in the process of changing drivers not to do this, one by one
> >>> > > so we can kill the BKL in the end.
> >>> > >
> >>> >
> >>> > I know we've discussed this before, but why wouldn't the following
> >>> > make more sense?
> >>> >  .llseek         = no_llseek,
> >>>
> >>> That's one of the possible solutions. Assigning it to generic_file_llseek
> >>> also gets rid of the BKL but keeps the current behaviour (calling seek
> >>> returns success without having an effect, no_llseek returns -ESPIPE),
> >>> while calling nonseekable_open has the other side-effect of making
> >>> pread/pwrite fail with -ESPIPE, which is more consistent than
> >>> only failing seek.
> >>>
> >>
> >> OK, so how about the patch below (on top of Thadeu's patch)?
> >>
> >> --
> >> Dmitry
> >>
> >> Input: uinput - use nonseekable_open
> >>
> >> Seeking does not make sense for uinput so let's use nonseekable_open
> >> to mark the device non-seekable.
> >>
> >> Signed-off-by: Dmitry Torokhov <dtor@...l.ru>
> >> ---
> >>
> >>  drivers/input/misc/uinput.c |    7 +++++++
> >>  1 files changed, 7 insertions(+), 0 deletions(-)
> >>
> >>
> >> diff --git a/drivers/input/misc/uinput.c b/drivers/input/misc/uinput.c
> >> index 18206e1..7089151 100644
> >> --- a/drivers/input/misc/uinput.c
> >> +++ b/drivers/input/misc/uinput.c
> >> @@ -278,6 +278,7 @@ static int uinput_create_device(struct uinput_device *udev)
> >>  static int uinput_open(struct inode *inode, struct file *file)
> >>  {
> >>        struct uinput_device *newdev;
> >> +       int error;
> >>
> >>        newdev = kzalloc(sizeof(struct uinput_device), GFP_KERNEL);
> >>        if (!newdev)
> >> @@ -291,6 +292,12 @@ static int uinput_open(struct inode *inode, struct file *file)
> >>
> >>        file->private_data = newdev;
> >>
> >> +       error = nonseekable_open(inode, file);
> >> +       if (error) {
> >> +               kfree(newdev);
> >> +               return error;
> >> +       }
> >> +
> >>        return 0;
> >>  }
> >>
> >>
> >
> > Hmnn, if you look at nonseekable_open() it will always return 0. I
> > think you can just do the following.

It always returns 0 _now_ but I do not see any guarantees that it will
never ever return anything but 0. If somebody would provide such
garantee then we certainly would not need to handle errors.

> >
> > diff --git a/drivers/input/misc/uinput.c b/drivers/input/misc/uinput.c
> > index 18206e1..697c0a6 100644
> > --- a/drivers/input/misc/uinput.c
> > +++ b/drivers/input/misc/uinput.c
> > @@ -291,7 +291,7 @@ static int uinput_open(struct inode *inode, struct file *fil
> >
> >        file->private_data = newdev;
> >
> > -       return 0;
> > +       return nonseekable_open(inode, file);
> >  }
> >
> > Signed-off-by: John Kacur <jkacur@...hat.com>
> >
> 
> Btw, Thadeu Lima de Souza Cascardo should just combine that all into
> one patch, no point really in making two patches out of that.

I think these are 2 separate changes (the fact that nonseekable_open
also gets rid of BKL invocation is a side-effect), that is not
considering the fact that I already applied Thadeu's change and don't
want to rewind my public branch unless really necessary.

Thanks.

-- 
Dmitry
--
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