linux-kernel - Re: [PATCH] macintosh: Explicitly set llseek to no

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Date:	Wed, 21 Oct 2009 23:33:17 +0200 (CEST)
From:	John Kacur <jkacur@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Jonathan Corbet <corbet@....net>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linuxppc-dev@...abs.org, Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arnd Bergmann <arndbergmann@...glemail.com>
Subject: Re: [PATCH] macintosh: Explicitly set llseek to no_llseek in
 ans-lcd



On Wed, 21 Oct 2009, Frederic Weisbecker wrote:

> On Wed, Oct 21, 2009 at 11:07:18PM +0200, John Kacur wrote:
> > From 0c2b412cdccf73bdeb19bb866bfe556942eaeca2 Mon Sep 17 00:00:00 2001
> > From: John Kacur <jkacur@...hat.com>
> > Date: Wed, 21 Oct 2009 23:01:12 +0200
> > Subject: [PATCH] macintosh: Explicitly set llseek to no_llseek in ans-lcd
> > 
> > Now that we've removed the BKL here, let's explicitly set lleek to no_llseek
> > 
> > Signed-off-by: John Kacur <jkacur@...hat.com>
> > ---
> >  drivers/macintosh/ans-lcd.c |    1 +
> >  1 files changed, 1 insertions(+), 0 deletions(-)
> > 
> > diff --git a/drivers/macintosh/ans-lcd.c b/drivers/macintosh/ans-lcd.c
> > index 4ae8ec9..a1a1bde 100644
> > --- a/drivers/macintosh/ans-lcd.c
> > +++ b/drivers/macintosh/ans-lcd.c
> > @@ -137,6 +137,7 @@ const struct file_operations anslcd_fops = {
> >  	.write		= anslcd_write,
> >  	.unlocked_ioctl	= anslcd_ioctl,
> >  	.open		= anslcd_open,
> > +	.llseedk	= no_llseek,
> 
> 
> llseedk? :)
> 
> 
> Should we better pushdown default_llseek to every to every
> file operations that don't implement llseek?
> I don't know how many of them don't implement llseek() though.
> 
> That said we can't continue anymore with this default attribution
> of default_llseek() on new fops.
> 

If you don't explicitly set it to no_llseek, you automatically get the
default_llseek, which uses the BKL. So if your driver doesn't need it, it 
is best to explicitly set it to no_llseek.

There is also a generic_file_llseek_unlocked, somewhat analogous to the 
unlocked_ioctls that you can use if you don't need to provide a full 
llseek yourself.
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