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

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Message-ID: <20090203153740.363d0a04@bike.lwn.net>
Date:	Tue, 3 Feb 2009 15:37:40 -0700
From:	Jonathan Corbet <corbet@....net>
To:	Matt Mackall <mpm@...enic.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Eric Dumazet <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, 03 Feb 2009 16:22:02 -0600
Matt Mackall <mpm@...enic.com> wrote:

> But that re-opens the question of what to do about poor Jon's quest.
>
> I got confused halfway through as he went from using a global fasync
> spinlock to a non-locked but atomic flag bit. Not sure why using a
> per-file or per-inode lock doesn't work for the fasync code.

No per-file lock because (1) there is resistance to growing struct
file, and (2) lockless algorithms are all the rage now.  Additionally,
solving the fasync-atomicity problem with locks requires the use of a
mutex (or the BKL) rather than a spinlock.  I suppose we could combine
a global f_flags lock with the set-FASYNC-in-fasync_helper() bits.

At this point Poor Jon sees a fork in the road as he contemplates the
future of his quest:

- Go with this patch set, perhaps with a bit of cleanup as suggested by
  Andrew.

- Go back to the global lock.

- Go away, leave the BKL in place, and wait for somebody smarter to
  attack the problem.

Any wise guidance would be most welcome...

jon
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