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Date: Wed, 21 Oct 2009 23:53:21 +0200 (CEST)
From: John Kacur <jkacur@redhat.com>
To: Frederic Weisbecker <fweisbec@gmail.com>
Subject: Re: [PATCH] macintosh: Explicitly set llseek to no_llseek in ans-lcd
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On Wed, 21 Oct 2009, Frederic Weisbecker wrote:

> On Wed, Oct 21, 2009 at 11:33:17PM +0200, John Kacur wrote:
> > > 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.
> 
> 
> Sure, that's the right thing to do.
> 
>  
> > 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.
> 
> 
> No problem with that. Setting no_llseek or generic_file_llseek_unlocked,
> depending on the context is the right thing to do.
> 
> What I'm wondering about concerns the future code that will have
> no llsek() implemented in their fops.
> 
> We can't continue to use default_llseek() for future code unless we
> want to continue these post reviews and fixes forever.
> 

I'm thinking that the simplier approach, would be to make the 
default_llseek the unlocked one. Then you only have to audit the drivers 
that have the BKL - ie the ones we are auditing anyway, and explicitly set 
them to the bkl locked llseek.

There might be a hidden interaction though between the non-unlocked 
variety of ioctls and default llseek though.