From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755381AbbBTXuS (ORCPT ); Fri, 20 Feb 2015 18:50:18 -0500 Received: from zeniv.linux.org.uk ([195.92.253.2]:33317 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754990AbbBTXuR (ORCPT ); Fri, 20 Feb 2015 18:50:17 -0500 Date: Fri, 20 Feb 2015 23:50:12 +0000 From: Al Viro To: Andrew Morton Cc: Konstantin Khlebnikov , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Dave Chinner Subject: Re: [PATCH] fs: avoid locking sb_lock in grab_super_passive() Message-ID: <20150220235012.GS29656@ZenIV.linux.org.uk> References: <20150219171934.20458.30175.stgit@buzz> <20150220150731.e79cd30dc6ecf3c7a3f5caa3@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150220150731.e79cd30dc6ecf3c7a3f5caa3@linux-foundation.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Feb 20, 2015 at 03:07:31PM -0800, Andrew Morton wrote: > - It no longer "acquires a reference". All it does is to acquire an rwsem. > > - What the heck is a "passive reference" anyway? It appears to be > the situation where we increment s_count without incrementing s_active. Reference to struct super_block that guarantees only that its memory won't be freed until we drop it. > After your patch, this superblock state no longer exists(?), Yes, it does. The _only_ reason why that patch isn't outright bogus is that we do only down_read_trylock() on ->s_umount - try to pull off the same thing with down_read() and you'll get a nasty race. Take a look at e.g. get_super(). Or user_get_super(). Or iterate_supers()/iterate_supers_type(), where we don't return such references, but pass them to a callback instead. In all those cases we end up with passive reference taken, ->s_umount taken shared (_NOT_ with trylock) and fs checked for being still alive. Then it's guaranteed to stay alive until we do drop_super(). I agree that the name blows, BTW - something like try_get_super() might have been more descriptive, but with this change it actually becomes a bad name as well, since after it we need a different way to release the obtained ref; not the same as after get_super(). Your variant might be OK, but I'd probably make it trylock_super(), to match the verb-object order of the rest of identifiers in that area... > so > perhaps the entire "passive reference" concept and any references to > it can be expunged from the kernel. Nope.