From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759343Ab3BKTWW (ORCPT ); Mon, 11 Feb 2013 14:22:22 -0500 Received: from hydra.sisk.pl ([212.160.235.94]:37241 "EHLO hydra.sisk.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757820Ab3BKTWV (ORCPT ); Mon, 11 Feb 2013 14:22:21 -0500 From: "Rafael J. Wysocki" To: Miklos Szeredi Cc: Pavel Machek , Goswin von Brederlow , Li Fei , len.brown@intel.com, mingo@redhat.com, peterz@infradead.org, biao.wang@intel.com, linux-pm@vger.kernel.org, fuse-devel@lists.sourceforge.net, linux-kernel@vger.kernel.org, chuansheng.liu@intel.com Subject: Re: Getting rid of freezer for suspend [was Re: [fuse-devel] [PATCH] fuse: make fuse daemon frozen along with kernel threads] Date: Mon, 11 Feb 2013 20:28:46 +0100 Message-ID: <2407765.5u9DGJxmKr@vostro.rjw.lan> User-Agent: KMail/4.9.5 (Linux/3.8.0-rc7; KDE/4.9.5; x86_64; ; ) In-Reply-To: References: <1360113112.17267.1.camel@fli24-HP-Compaq-8100-Elite-CMT-PC> <1660768.prnx6s70rD@vostro.rjw.lan> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="utf-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Monday, February 11, 2013 02:59:56 PM Miklos Szeredi wrote: > On Mon, Feb 11, 2013 at 1:08 PM, Rafael J. Wysocki wrote: > > On Monday, February 11, 2013 11:11:40 AM Miklos Szeredi wrote: > >> On Mon, Feb 11, 2013 at 12:31 AM, Rafael J. Wysocki wrote: > >> > On Sunday, February 10, 2013 07:55:05 PM Pavel Machek wrote: > >> > >> >> Well, from freezer you need: > >> >> > >> >> 1) user process frozen. > >> >> > >> >> 2) essential locks _not_ held so that block devices are still functional. > >> >> > >> >> > > > mmap... what is problem with mmap? For suspend, memory is powered, so > >> >> > > > you can permit people changing it. > >> >> > > > >> >> > > Suppose mmap is used to make the registers of some device available to user > >> >> > > space. Yes, that can happen. > >> >> > >> >> "Don't do it, then". Yes, can happen, but hopefully is not too common > >> >> these days. [And... freezer doing 1) but not 2) would be enough to > >> >> handle that. Freezer doing 1) but not 2) would also be simpler...] > >> > > >> > Again, I'm not sure what you mean. > >> > > >> > Are you trying to say that it would be OK to freeze user space tasks in > >> > the D state? > >> > >> I think that's what Pavel is saying. Processes in D state sleeping > >> on non-device mutexes _are_ actually OK to freeze. And that would > >> nicely solve the fuse freeze problem. > > > > That's potentially deeadlock-prone, because a task waiting for mutex X may > > very well be holding mutex Y, so if there's another task waiting for mutex Y, > > it needs to be frozen at the same time. > > > >> The only little detail is how do we implement that... > > > > This means the only way I can see would be to hack the mutex code so that the > > try_to_freeze() was called for user space tasks after the > > sched_preempt_enable_no_resched() before schedule(). > > > > That shouldn't be a big deal performance-wise, because we are in the slow > > path anyway then. I'm not sure if Peter Z will like it, though. > > > > Moreover, a task waiting for a mutex may be holding a semaphore or be > > participating in some other mutual-exclusion mechanism, so we'd need to > > address them all. Plus, as noted by Pavel, freezing those things would make > > it difficult to save hibernation images to us. > > Well, as a first step I could cook up a patch that adds a flag to the > mutex indicating that it's freezable. Fuse would mark its mutexes > (and the mutexes that VFS uses on its behalf) as freezable. That way > we don't interfere with hibernation, except if hibernation uses fuse > but that's a very special case. Yes, that may be worth a shot. :-) Thanks, Rafael -- I speak only for myself. Rafael J. Wysocki, Intel Open Source Technology Center.