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

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Message-Id: <200707051048.46202.oliver@neukum.org>
Date:	Thu, 5 Jul 2007 10:48:45 +0200
From:	Oliver Neukum <oliver@...kum.org>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	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

Am Donnerstag, 5. Juli 2007 schrieb Miklos Szeredi:
> > > > > > 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 ;)

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.

	Regards
		Oliver

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