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

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Message-Id: <E1I6Mtu-000058-00@dorka.pomaz.szeredi.hu>
Date:	Thu, 05 Jul 2007 10:41:22 +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

> > > > > 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.
> 
> Then this is a problem specific to fuse. You should teach fuse to block
> suspension while such operations are being performed.

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 ;)

Miklos
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