Re: Re: [PATCH] Remove process freezer from suspend to RAM pathway

Miklos Szeredi
2007-07-05T09:31:39Z

> > > > > 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