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

[Jonathan Corbet]

On Tue, 3 Feb 2009 14:53:46 -0800
Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> 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
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