linux-kernel - Re: [linux-pm] Re: [PATCH] Remove process freezer from suspend to RAM pathway

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Date:	Thu, 05 Jul 2007 11:31:39 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	pavel@....cz
CC:	miklos@...redi.hu, oliver@...kum.org, paulus@...ba.org,
	stern@...land.harvard.edu, johannes@...solutions.net, rjw@...k.pl,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	mjg59@...f.ucam.org, benh@...nel.crashing.org
Subject: Re: [linux-pm] Re: [PATCH] Remove process freezer from suspend to RAM pathway

> > > > > I have discussed the benefits elsewhere.  As for the deadlocks -- do 
> > > > > you still observe them if you use the version of the freezer which 
> > > > > doesn't freeze kernel threads?
> > > > 
> > > > In general the only way to guarantee there are no deadlocks is to
> > > > construct the graph of dependencies between tasks.  Those dependencies
> > > > are not in practice observable from outside the tasks, so it is
> > > > virtually impossible to construct the graph.
> > > 
> > > In which way can user space tasks depend on each other in a way that
> > > allows a them members of that cycle to be in uninterruptible sleep?
> > 
> >  - process A calls rename() on a fuse fs
> >  - process B, the fuse server, starts to process the rename request
> >  - process B is frozen before it can reply
> > 
> > Now process A is unfreezable.  We cannot make rename() restartable,
> > hence it cannot be interruptible.
> 
> Yes, we are claiming fuse is very special in this regard, and perhaps
> even broken.
> 
> Let's see. If I SIGSTOP the fuse server, I can get unrelated tasks
> unkillable (even for SIGKILL!) forever.

Actually fuse allows SIGKILL, because it's always fatal, and the
syscall may not be restarted.

> That's very special, and maybe even a FUSE bug. And that is also
> what makes FUSE special w.r.t. s2ram.

What makes fuse special is that some file operations are synchronous
and non-restartable.  That's just how the UNIX filesystem API works
and is hardly a bug in fuse.

> So no, you can't claim "FUSE is just IPC". It is very special IPC.

I did say it's special.  Sure, it has some "interesting" properties,
and with a bit of malice you can do very ugly things with it.  If you
are interested, read Documentation/filesystems/fuse.txt, especially
the "Tricky deadlock" section ;)

Miklos
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