Hi, On Sunday, 18 March 2007 00:21, Johannes Berg wrote: > Hi, > > When on vacation I was doing a bit more stuff w/o battery and noticed > that I after my patches to implement proper pm_ops for PMU-based > machines the machine would hang during suspend. Now on the way back I'm > sitting in a train and it's working again. Couldn't really be the lower > atmospheric pressure causing it ;) So I looked again and realised that > when writing those patches I thought doing sys_sync() in the ->prepare > callback would be a good idea. It is, as it turns out, not such a great > idea at all, right now anyway. Well, it's not a good idea indeed. > When laptop mode is enabled [1], xfs will, upon getting a sync, try to > wake up one of it's tasks and then wait for something that task does (as > far as I can tell.) Even though I carry the patch to make xfs's > workqueues non-freezable this event doesn't seem to happen when > processes are frozen and so the system hangs. Not sure why that is but > it isn't really the point right now, read on. XFS uses more kernel threads that are frozen (see http://lxr.free-electrons.com/source/fs/xfs/linux-2.6/xfs_buf.c?a=x86_64#1700) > I have now removed the call to sys_sync() to double-verify and that has > indeed cured the problem, but the bigger question really is whether it > should be safe to call it or not [2], especially given that it would > make sense to call it when processes can no longer submit IO. We call sys_sync() in freeze_processes() _before_ kernel threads are frozen. I don't think it's safe to call it any time later before the kernel threads are thawed. > The real thing we should be doing is think about a whole range of things > you can do in the kernel and then think about whether they should be > allowed to be done in the various stages. A lot of these things are at > least somewhat documented for early boot stages (and you get into > trouble when you do it wrong) but it seems that many developers never > suspend their machines True, but they should do this. > so this needs to be documented in a more explicit > way rather than just breaking when you try (bootup is part of regular > testing routine in an obvious way that suspend isn't) > > Yup, this is a lot of hard work, but it could be done gradually as we > run into these problems. > > Back to the specific sys_sync() issue, what do people feel is > appropriate? Should XFS get into even more trouble? It seems not to be > able to guarantee full sync when processes are frozen right now, at > least not the way sync is currently implemented. See above. I think at least some filesystems use freezable kernel threads and will block if you ask them to sync after these threads have been frozen. Greetings, Rafael _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
