From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754122AbZBDBA5 (ORCPT ); Tue, 3 Feb 2009 20:00:57 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752472AbZBDBAq (ORCPT ); Tue, 3 Feb 2009 20:00:46 -0500 Received: from movementarian.org ([79.99.65.163]:41266 "EHLO movementarian.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752470AbZBDBAq (ORCPT ); Tue, 3 Feb 2009 20:00:46 -0500 Date: Tue, 3 Feb 2009 20:00:36 -0500 From: wli@movementarian.org To: Andrew Morton Cc: Jonathan Corbet , linux-kernel@vger.kernel.org, andi@firstfloor.org, oleg@redhat.com, viro@ZenIV.linux.org.uk, davidel@xmailserver.org, davem@davemloft.net, hch@lst.de, linux-api@vger.kernel.org, mpm@selenic.com, alan@lxorguk.ukuu.org.uk Subject: Re: [PATCH 2/4] Convert epoll to a bitlock Message-ID: <20090204010036.GA10229@movementarian.org> References: <1233598811-6871-1-git-send-email-corbet@lwn.net> <1233598811-6871-3-git-send-email-corbet@lwn.net> <20090203133942.2ecec281.akpm@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090203133942.2ecec281.akpm@linux-foundation.org> User-Agent: Mutt/1.4.2.2i Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2 Feb 2009 11:20:09 -0700 Jonathan Corbet 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