From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric Dumazet Subject: Re: [patch 1/6] fs: icache RCU free inodes Date: Tue, 09 Nov 2010 17:21:38 +0100 Message-ID: <1289319698.2774.16.camel@edumazet-laptop> References: <20101109124610.GB11477@amd> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Nick Piggin , Al Viro , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org To: Linus Torvalds Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org Le mardi 09 novembre 2010 =C3=A0 08:02 -0800, Linus Torvalds a =C3=A9cr= it : > On Tue, Nov 9, 2010 at 4:46 AM, Nick Piggin wrote= : > > So here is the inode RCU code. It's obviously not worth doing until= the > > actual rcu-walk path walking is in, but I'd like to get opinions on= it. > > It would be nice to merge it in Al's tree at some point, though. >=20 > Remind me why it wasn't sufficient to just use SLAB_DESTROY_BY_RCU? >=20 > Especially if we still lock things for the actual (few) inode list > operations, the added complexity of actually freeing _individual_ > inodes by RCU seems to be a bad thing. >=20 > The only thing we care about is the pathname walk - there are no othe= r > inode operations that are common enough to worry about. And the only > thing _that_ needs is the ability to look at the inode under RCU, and > SLAB_DESTROY_BY_RCU should be entirely sufficient for that. >=20 > But we had some discussion about this long ago, and I may have > forgotten some of the context. >=20 David Chinner sent a patch using SLAB_DESTROY_BY_RCU You can see problems using this fancy thing : - Need to use slab ctor() to not overwrite some sensitive fields of reused inodes. (spinlock, next pointer) - Fancy algo to detect an inode moved from one chain to another. Lookup= s should be able to detect and restart their loop. - After a match, need to get a stable reference on inode (lock), then recheck the keys to make sure the target inode is the right one. http://www.gossamer-threads.com/lists/linux/kernel/1298710 A bit tricky, but doable IMHO, especially if covering only the main lookup. The seldom used can still get a (spin)lock.