Re: [ 06/23] [Suspend2] Disable usermode helper invocations when the freezer is on.

[Nigel Cunningham <nigel@suspend2.net>]

Hi.

On Tuesday 31 January 2006 20:58, Rafael J. Wysocki wrote:
> Hi,
> > > >
> > > > +	if (freezer_is_on())
> > > > +		return 0;
> > > > +
> > > >  	if (path[0] == '\0')
> > > >  		return 0;
> > >
> > > Disabling the usermode helper while freeze_processes() is executed
> > > seems to be a good idea to me, but I think it should be done with a
> > > mutex or something like that.
> >
> > With the refrigerator code you guys are using at the moment, ouldn't that
> > result in deadlocks when we later try to freeze the process in
> > preparation for the atomic restore? (Or perhaps you don't freeze
> > processes at that point?)
>
> I'm not sure what you mean.  I said "mutex" because you seem to have a race
> here (the freezer may be started right after the freezer_is_on() check). 
> IMO the freezer should disable the invocations of new usermode helpers and
> wait util all of the already running helpers are finished.  For this
> purpose two variables would be needed and a lock.

Sorry. Being a bit thick.

I wasn't worried about already-running usermode helpers (or about-to-run 
helpers either) because the spawned processes would either complete or be 
caught be the usual freezing code. My concern was more that new invocations 
of this path don't leave us with unfrozen processes hanging around. That 
said, I think you have a valid point about letting existing helpers run to 
completion. It does make me concerned though about the possibility of a 
long-lived helper (not that I know that there really are such things at the 
moment). Do you think that needs to be taken as a genuine concern? If so, I 
guess we then need to make freezing a four stage process:

1. Stop new usermodehelpers from starting & let existing ones run to 
completion.
2. Freeze userspace.
3. Freezer bdevs.
4. Freezer kernel space.

Regards,

Nigel
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