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Date: Mon, 11 Feb 2013 20:28:46 +0100 From: "Rafael J. Wysocki" <rjw@...k.pl> To: Miklos Szeredi <miklos@...redi.hu> Cc: Pavel Machek <pavel@....cz>, Goswin von Brederlow <goswin-v-b@....de>, Li Fei <fei.li@...el.com>, len.brown@...el.com, mingo@...hat.com, peterz@...radead.org, biao.wang@...el.com, linux-pm@...r.kernel.org, fuse-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org, chuansheng.liu@...el.com Subject: Re: Getting rid of freezer for suspend [was Re: [fuse-devel] [PATCH] fuse: make fuse daemon frozen along with kernel threads] On Monday, February 11, 2013 02:59:56 PM Miklos Szeredi wrote: > On Mon, Feb 11, 2013 at 1:08 PM, Rafael J. Wysocki <rjw@...k.pl> 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 <rjw@...k.pl> 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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