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

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Date:	Thu, 05 Jul 2007 12:14:07 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	oliver@...kum.org
CC:	miklos@...redi.hu, paulus@...ba.org, stern@...land.harvard.edu,
	johannes@...solutions.net, rjw@...k.pl,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	pavel@....cz, mjg59@...f.ucam.org, benh@...nel.crashing.org
Subject: Re: [linux-pm] Re: [PATCH] Remove process freezer from suspend to RAM pathway

> > > > And teach VFS to block suspension, while waiting on a mutex held by
> > > > another process performing a fuse operation.
> > > > 
> > > > I can already hear the beautiful praise from Al Viro at the sight of
> > > > that ;)
> > > 
> > > There is that.
> > > 
> > > OK, bite the bullet. Tasks involved in fuse are special. Give them a flag
> > > and teach the freezer to put them on ice only after all other task are
> > > frozen. In a way they are kernel, there's no use denying that.
> > 
> > And flag every other process, that the flagged process is
> > communicating with?  How are you proposing to do that?
> > 
> > Quoting Paul:
> > 
> > "1. The freezer cannot be guaranteed deadlock-free without constructing
> >    a dependency graph between tasks (both user and kernel), which is
> >    virtually impossible since the dependencies are not externally
> >    observable."
> 
> A deadlock requires that the circular wait is uninterruptible. Normal IPC
> isn't.
> 
> What are you doing in the userland portions of fuse? Some kind of IPC
> with other tasks?

Anything, writing to a file, writing to shared memory, sending things
over the network.  There's no limit to what a filesystem daemon may
do.  It's a perfectly ordinary unprivileged userspace process.  And
this is a feature not a bug.

> There is a limit to which you can push kernel functionality into
> user space.

Limiting what a userspace filesystem can do would defeat the whole
purpose of the bloody thing.  This is not negotiable ;)

Miklos
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