From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753390AbZK3Wtd (ORCPT ); Mon, 30 Nov 2009 17:49:33 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752265AbZK3Wtd (ORCPT ); Mon, 30 Nov 2009 17:49:33 -0500 Received: from smtp1.linux-foundation.org ([140.211.169.13]:33298 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751964AbZK3Wtc (ORCPT ); Mon, 30 Nov 2009 17:49:32 -0500 Date: Mon, 30 Nov 2009 14:49:10 -0800 (PST) From: Linus Torvalds X-X-Sender: torvalds@localhost.localdomain To: Thomas Gleixner cc: Peter Zijlstra , Ingo Molnar , Christoph Hellwig , Nick Piggin , Linux Kernel Mailing List Subject: Re: [rfc] "fair" rw spinlocks In-Reply-To: Message-ID: References: <20091123145409.GA29627@wotan.suse.de> <20091130100041.GA29610@infradead.org> <20091130174638.GA9782@elte.hu> <1259616429.26472.499.camel@laptop> User-Agent: Alpine 2.00 (LFD 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 30 Nov 2009, Thomas Gleixner wrote: > > I'm aware of that. The number of places where we read_lock > tasklist_lock is 79 in 36 files right now. That's not a horrible task > to go through them one by one and do a case by case conversion with a > proper changelog. That would only leave the write_lock sites. The write_lock sites should be fine, since just changing them to a spinlock should be 100% semantically equivalent - except for the lack of interrupt disable. And the lack of interrupt disable will result in a nice big deadlock if some interrupt really does take the spinlock, which is much easier to debug than a subtle race that would get the wrong read value. > We can then either do the rw_lock to spin_lock conversion or keep the > rw_lock which has no readers anymore and behaves like a spinlock for a > transition time so reverts of one of the read_lock -> rcu patches > could be done to debug stuff. So as per the above, I wouldn't worry about the write lockers. Might as well change it to a spinlock, since that's what it will act as. It's not as if there is any chance that the spinlock code is subtly buggy. So the only reason to keep it as a rwlock would be if you decide to do the read-locked cases one by one, and don't end up with all of them converted. Which is a reasonable strategy too, of course. We don't _have_ to convert them all - if the main problem is some starvation issue, it's sufficient to convert just the main read-lock cases so that writers never get starved. But converting it all would be nice, because that whole write_lock_irq(&tasklist_lock); to spin_lock(&tasklist_lock); conversion would likely be a measurable performance win. Both because spinlocks are fundamentally faster (no atomic on unlock), and because you get rid of the irq disable/enable. But in order to get there, you'd have to convert _all_ the read-lockers, so you'd miss the opportunity to only convert the easy cases. Linus