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

Jonathan Corbet
2009-02-03T23:20:04Z

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. 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?

> 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?

jon