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

Jonathan Corbet
2009-02-03T22:37:52Z

On Tue, 03 Feb 2009 16:22:02 -0600
Matt Mackall <mpm@selenic.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