From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753250AbdLHSOv (ORCPT ); Fri, 8 Dec 2017 13:14:51 -0500 Received: from bombadil.infradead.org ([65.50.211.133]:42498 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751944AbdLHSOt (ORCPT ); Fri, 8 Dec 2017 13:14:49 -0500 Date: Fri, 8 Dec 2017 10:14:38 -0800 From: Matthew Wilcox To: "Theodore Ts'o" , Dave Chinner , Matthew Wilcox , Ross Zwisler , Jens Axboe , Rehas Sachdeva , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, linux-nilfs@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-xfs@vger.kernel.org, linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org, mingo@kernel.org, byungchul.park@lge.com Subject: Re: Lockdep is less useful than it was Message-ID: <20171208181438.GA6406@bombadil.infradead.org> References: <20171206004159.3755-73-willy@infradead.org> <20171206012901.GZ4094@dastard> <20171206020208.GK26021@bombadil.infradead.org> <20171206031456.GE4094@dastard> <20171206044549.GO26021@bombadil.infradead.org> <20171206084404.GF4094@dastard> <20171206140648.GB32044@bombadil.infradead.org> <20171207160634.il3vt5d6a4v5qesi@thunk.org> <20171207223803.GC26792@bombadil.infradead.org> <20171208152717.fx5w66wvyrfx6vrz@thunk.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171208152717.fx5w66wvyrfx6vrz@thunk.org> User-Agent: Mutt/1.9.1 (2017-09-22) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Dec 08, 2017 at 10:27:17AM -0500, Theodore Ts'o wrote: > So if you are adding complexity to the kernel with the argument, > "lockdep will save us", I'm with Dave --- it's just not a believable > argument. I think that's a gross misrepresentation of what I'm doing. At the moment, the radix tree actively disables the RCU checking that enabling lockdep would give us. It has to, because it has no idea what lock protects any individual access to the radix tree. The XArray can use the RCU checking because it knows that every reference is protected by either the spinlock or the RCU lock. Dave was saying that he has a tree which has to be protected by a mutex because of where it is in the locking hierarchy, and I was vigorously declining his proposal of allowing him to skip taking the spinlock. And yes, we have bugs today that I assume we only stumble across every few billion years (or only on alpha, or only if our compiler gets more aggressive) because we have missing rcu_dereference annotations.