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

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Date:	Tue, 3 Feb 2009 20:00:36 -0500
From:	wli@...ementarian.org
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Jonathan Corbet <corbet@....net>, 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, mpm@...enic.com,
	alan@...rguk.ukuu.org.uk
Subject: Re: [PATCH 2/4] Convert epoll to a bitlock

On Mon,  2 Feb 2009 11:20:09 -0700 Jonathan Corbet <corbet@....net> wrote:
>> Matt Mackall suggested converting epoll's ep_lock to a bitlock as a way of
>> saving space in struct file.  This patch makes that change.

On Tue, Feb 03, 2009 at 01:39:42PM -0800, Andrew Morton wrote:
> hrm.  bit_spin_lock() makes people upset (large penguiny people).  iirc
> it doesn't have all the correct/well-understood memory/compiler
> ordering semantics which spinlocks have.  And lockdep doesn't know about
> it.

ISTR the memory/compiler ordering semantics bits coming up when it was
still pte_chain_lock(), but not the entirety of it. I think
smp_mb__after_clear_bit() and/or smp_mb__before_clear_bit() turned out
to be needed in the unlock function, and they're there now with
clear_bit_unlock() et al. lockdep I'm less sure about, but suspect
the objects with bit locks embedded in them are too numerous to feasibly
track, e.g. there may be several bh's per-page, and how numerous they
are tends to be why they're using bit locks in the first place.

-- wli
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