From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nick Piggin Subject: Re: [PATCH 02/46] fs: d_validate fixes Date: Wed, 8 Dec 2010 17:59:55 +1100 Message-ID: <20101208065955.GA14846@amd> References: <0fff695735c9b652a3f63b8480686c64811e89d0.1290852958.git.npiggin@kernel.dk> <20101208015344.GE29333@dastard> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Nick Piggin , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org To: Dave Chinner Return-path: Content-Disposition: inline In-Reply-To: <20101208015344.GE29333@dastard> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Wed, Dec 08, 2010 at 12:53:44PM +1100, Dave Chinner wrote: > On Sat, Nov 27, 2010 at 08:44:32PM +1100, Nick Piggin wrote: > > d_validate has been broken for a long time. > > > > kmem_ptr_validate does not guarantee that a pointer can be dereferenced > > if it can go away at any time. Even rcu_read_lock doesn't help, because > > the pointer might be queued in RCU callbacks but not executed yet. > > > > So the parent cannot be checked, nor the name hashed. The dentry pointer > > can not be touched until it can be verified under lock. Hashing simply > > cannot be used. > > > > Instead, verify the parent/child relationship by traversing parent's > > d_child list. It's slow, but only ncpfs and the destaged smbfs care > > about it, at this point. > > I'd drop the previous revert patch and just convert the RCU hash > traversal straight to the d_child traversal code you introduce here. > This is a much better explanation of why the d_validate mechanism > needs to be changed, and the revert is really an unnecessary extra > step... Has to be backported, though. Patch that is to be reverted obviously adds more brokenness and is a good example that you cannot dget() under rcu read protection even if the rest of the surrounding function is bugfree. I wouldn't have thought it's a big deal.