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

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Message-Id: <200707060916.16368.rjw@sisk.pl>
Date:	Fri, 6 Jul 2007 09:16:15 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Pavel Machek <pavel@....cz>
Cc:	Miklos Szeredi <miklos@...redi.hu>, oliver@...kum.org,
	paulus@...ba.org, stern@...land.harvard.edu,
	johannes@...solutions.net, 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

On Friday, 6 July 2007 00:04, Pavel Machek wrote:
> Hi!
> 
> > > > > > > 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.
> 
> Okay, and you should handle refrigerator in the same paths where you
> handle SIGKILL. Just add try_to_freeze() there...

In fact the problem is more complicated than that, because some tasks may
be waiting on VFS locks related to FUSE.

Greetings,
Rafael


-- 
"Premature optimization is the root of all evil." - Donald Knuth
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