From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757499Ab0LHBRF (ORCPT ); Tue, 7 Dec 2010 20:17:05 -0500 Received: from bld-mail20.adl6.internode.on.net ([150.101.137.105]:35685 "EHLO mail.internode.on.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1757474Ab0LHBRD (ORCPT ); Tue, 7 Dec 2010 20:17:03 -0500 Date: Wed, 8 Dec 2010 12:16:56 +1100 From: Dave Chinner To: Nick Piggin Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 01/46] Revert "fs: use RCU read side protection in d_validate" Message-ID: <20101208011656.GD29333@dastard> References: <2d1d8ffc4acea8b6c4e5b58bb1653b3f0e7071e2.1290852958.git.npiggin@kernel.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2d1d8ffc4acea8b6c4e5b58bb1653b3f0e7071e2.1290852958.git.npiggin@kernel.dk> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Nov 27, 2010 at 08:56:03PM +1100, Nick Piggin wrote: > This reverts commit 3825bdb7ed920845961f32f364454bee5f469abb. > > Patch is broken, you can't dget() without holding any locks! I believe you can - for the same reasons we can take a reference to an inode without holding the inode_lock. That is, as long as the caller already holds an active reference to the dentry, dget() can be used to take another reference without needing the dcache_lock. Such usage appears to be described in the comment above dget() and there's a BUG_ON() in dget() to catch callers that don't already have an active reference. An example of a valid unlocked dget(): d_alloc() does an unlocked dget() to take a reference to the parent dentry which we already are guaranteed to have a reference to. As to d_validate() - it depends on the caller behaviour as to whether the unlocked dget() is valid or not. From a cursory check of the NCP and SMB readdir caches, both appear to hold an active reference to the dentry it is passing to d_validate(). If that is the case then there is nothing wrong with the way d_validate uses dget(). Can someone with more SMB/NCP expertise than me validate the use of cached dentries? Cheers, Dave. -- Dave Chinner david@fromorbit.com