Re: [PATCH 2/4] Convert epoll to a bitlock

Andrew Morton
2009-02-03T23:30:33Z

On Tue, 3 Feb 2009 16:19:31 -0700
Jonathan Corbet <corbet@lwn.net> wrote:

> On Tue, 3 Feb 2009 14:53:46 -0800
> Andrew Morton <akpm@linux-foundation.org> wrote:
>
> > Well. We _could_ whack part of this nut with my usual hammer: protect
> > f_flags with file->f_dentry->d_inode->i_lock. IIRC there was some
> > objection to that - performance?
>
> Andi has objected to the addition of locks, but i_lock is maybe
> sufficiently dispersed to pass muster there.

Hope so.

I'd wrap it in a lock_file_flags(file*) thing so we can change it later
on (add a lock to struct file, take a global, lock, etc).

> I had an instinctive
> reaction to using a lock which is three pointers away, but I can get
> over that. I'll admit a bit of ignorance, though: if a given struct
> file exists, do we know for sure that file->f_dentry->d_inode exists?

It should. A NULL ->d_inode especially signifies a negative dentry.

> > One problem here seems to be that we're trying to change multiple
> > things at the same time. We can blame the BKL for that.
> >
> > Can we break the problem into manageable chunks? Your patchset did
> > that, I guess. What were those chunks again? ;)
>
> I'm not really sure how to break it down any further. If we take the
> i_lock approach, the chunks would be something like:
>
> 1) Use i_lock to protect accesses to f_flags. This would enable some
> BKL usage to be removed, but would not fix fasync.
>
> 2) Move responsibility for the FASYNC bit into ->fasync(), with
> fasync_helper() doing it in almost all situations. The remaining
> BKL usage would then go away.
>
> 3) The same optional fasync() return values cleanup.
>
> Make sense?

yup.

If the ->i_lock think is no good then we can trivially switch over to a
global lock. Heck, we could even go back to lock_kernel() ;)