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

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Message-ID: <20090203161931.4054a25e@bike.lwn.net>
Date:	Tue, 3 Feb 2009 16:19:31 -0700
From:	Jonathan Corbet <corbet@....net>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	mpm@...enic.com, dada1@...mosbay.com, linux-kernel@...r.kernel.org,
	andi@...stfloor.org, oleg@...hat.com, viro@...IV.linux.org.uk,
	davidel@...ilserver.org, davem@...emloft.net, hch@....de,
	linux-api@...r.kernel.org, alan@...rguk.ukuu.org.uk
Subject: Re: [PATCH 2/4] Convert epoll to a bitlock

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