On Thursday, 5 July 2007 02:43, Paul Mackerras wrote:
quoted text
> Miklos Szeredi writes:
> 
> > OK, let me summarize the situation as I see it now: there are two
> > camps, the pro-freezers and the anti-freezers.
> > 
> > Pro-freezers say:
> > 
> >   - don't remove the freezer, otherwise we'll have to deal with
> >     numerous problems in drivers
> > 
> > Anti-freezers say:
> > 
> >   - let's remove the freezer, which causes numerous problems
> > 
> > Alan summerized the pro-freezer arguments well I think.  What are the
> > anti-freezer arguments then?
> 
> 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.

I don't agree with that.

The freezer only fails to handle uninterruptible tasks, so we need to take
the situations in which an uninterruptible task waits for a frozen task into
consideration.  Now, if both tasks are from the user land, this is highly
unusual.

quoted text
> 2. As a consequence of (1), we try to make a crude approximation of
>    the graph by saying "only kernel threads that want to be frozen
>    will be frozen" or some other similar statement.

No.  The rule is that kernel threads should not be freezable, but there are
some for which that is useful.

quoted text
> 3. However, (2) means that we can no longer guarantee that drivers
>    will not get any I/O requests after their suspend method has been
>    called, and therefore the freezer fails in its main objective.

This is a very general statement.  Can you please give some examples?

quoted text
> 4. We have an existence proof that reliable suspend can be achieved
>    without the freezer.

No.  We only know that it might work if the nonboot CPUs are disabled before
suspending devices, which is not the case in the generic suspend code.

quoted text
> To summarize, the argument is that the freezer is deadlock-prone and
> ineffective.

I remain unconvinced. ;-)

Greetings,
Rafael


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