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

[Dmitry Torokhov]

On Mon, Feb 01, 2010 at 10:50:25PM +0100, John Kacur wrote:
> On Mon, Feb 1, 2010 at 10:21 PM, Dmitry Torokhov
> <dmitry.torokhov@xxxxxxxxx> wrote:
> > 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@xxxxxxxxxx> wrote:
> >> > On Sun, Jan 31, 2010 at 6:29 AM, Dmitry Torokhov
> >> > <dmitry.torokhov@xxxxxxxxx> 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@xxxxxxx>
> >> >> ---
> >> >>
> >> >>  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.
> 
> Well, all it's doing is changing the f_mode. If anyone ever changes
> that function
> to return anything other than 0 it will be their responsibility to go
> fix all the
> uses of it.

No, not really.

> If you do a git grep of nonseekable_open, you'll see that this
> is a very common paradigm. (to return 0).

The reason for nonseekable_open return 0 is so that you can plug it
directly into fsops. The fact that many users abuse that and do:

	return nonseekable_open(indoe, file);

when doing:

	nonseekable_open(indoe, file);
	return 0;

would not introduce any complexity if they dont want to handle errors at
this time, and would be much safer (and one could mark
nonseekable_open() __must_check down the road if it is ever changed
to actually fail), does not validate such practice in any way.

> It makes your code shorter,
> and more readable. Plus, you are writing speculative code based on
> what might exist in the future?

No, I try to write the code that handles errors from functions that
could return errors even if current implementation does not do that.

> Also, then should uinput_release be called?
> If it is called will kfree be called twice on the same memory. If it
> isn't called, is
> that a problem because you've already done most of the work that requires
> a call to uinput_destroy_device ?

Why would release be called if open failed?

-- 
Dmitry
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/