From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756650Ab2ADSAi (ORCPT ); Wed, 4 Jan 2012 13:00:38 -0500 Received: from cantor2.suse.de ([195.135.220.15]:59076 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750884Ab2ADSAg (ORCPT ); Wed, 4 Jan 2012 13:00:36 -0500 Date: Wed, 4 Jan 2012 19:00:33 +0100 From: Jan Kara To: Dave Chinner Cc: Al Viro , Jan Kara , Stephen Rothwell , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Mikulas Patocka , Linus Torvalds Subject: Re: linux-next: build failure after merge of the vfs tree Message-ID: <20120104180033.GE28907@quack.suse.cz> References: <20120103124331.f0f0043f8ca464c9ff13f4d3@canb.auug.org.au> <20120103133942.GC31457@quack.suse.cz> <20120103144531.GA23916@ZenIV.linux.org.uk> <20120104021754.GD23916@ZenIV.linux.org.uk> <20120104025020.GW23662@dastard> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120104025020.GW23662@dastard> 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 Wed 04-01-12 13:50:20, Dave Chinner wrote: > On Wed, Jan 04, 2012 at 02:17:54AM +0000, Al Viro wrote: > > I'm still not > > sure about ->statfs(), BTW - any input on that would be welcome. Can > > it end up blocked on a frozen fs until said fs is thawed? > > I don't see why this should ever happen - ->statfs has to work on > read-only filesystems so shoul dnot be modifying state, and hence > should never need to care about the frozen state of the superblock. Well, I'm also not aware of a filesystem where ->statfs would wait on frozen filesystem. Just note that e.g. for stat(2) frozen filesystem and RO filesystem *are* different because of atime updates. So stat(2) can block on frozen fs because of atime update while on RO filesystem it is just fine. > So from a ->statfs POV, a frozen filesystem should look just like a > read-only filesystem. If frozen filesystems are holding locks that > ->statfs can block on until the filesystem us thawed, then I'd > consider that a bug in the filesystem freeze implementation.... In an ideal world yes. Practically, current freeze code has races (vfs_check_frozen() is a totally racy check) which can leave processes waiting for frozen fs with filesystem locks held. I believe we need something like mnt_want_write()/mnt_drop_write() for freezing code in ->page_mkwrite() and ->write_begin/->write_end. I'm now looking into how to do that in the best way. > > to convert ustat(2) to "wait for thaw" semantics (should be interruptible, > > BTW) or document that ->statfs() is not allowed to wait for thawing. > > It's far too subtle to leave undocumented... > > The latter, IMO. Agreed. Honza -- Jan Kara SUSE Labs, CR