Messages in this thread | | | Subject | Re: [linux-pm] Freezer: Don't count threads waiting for frozen filesystems. | From | Miklos Szeredi <> | Date | Wed, 29 Oct 2008 16:50:49 +0100 |
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On Wed, 29 Oct 2008, Alan Stern wrote: > On Wed, 29 Oct 2008, Miklos Szeredi wrote: > > I did a random sampling of ->suspend() callbacks, and they don't seem > > to be taking mutexes. Does that happen at all? > > It does, particularly among drivers that do runtime PM, which is > becoming more and more important. > > Besides, suspend has to synchronize with I/O somehow. Right now that > is handled by making suspend wait until no tasks are doing I/O (because > they are all frozen).
What about async I/O?
> If you allow tasks to be frozen at more or less > arbitrary times, while holding arbitrary locks, then you may end up > freezing a task that's in the middle of I/O. That should certainly > block the suspend (not to mention messing up the I/O operation).
What is the middle of I/O? Depending the type of I/O it could be messed up regardless of whether tasks happen to be in userspace or not (e.g. printing).
And some types of I/O are already mostly decoupled from userspace (file I/O, networking), so the userspace freezing shoudln't make any difference.
> > Did anybody ever try modifying the freezer for suspend (not > > hibernate), so that it allows tasks not in running state to freeze? > > If not, I think that's an experiment worth doing. > > What happens if the reason the task isn't running is because it's > waiting for I/O to complete? I just don't think this can be made to > work.
Don't know. I've never written a driver, and I'm not familiar with runtime PM, etc. So I can't come up with a detailed design for solving the freezer issues there.
But I do think that the solution does not lie in "fixing" the VFS.
Miklos
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