Re: [RFC][PATCH -mm] Freezer: Handle uninterruptible tasks

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Saturday, 7 July 2007 13:31, Pavel Machek wrote:
> Hi!
> 
> > > I don't think we can do that. I suspect rename looks like:
> > > 
> > > 	write directory entry in source
> > > A)	(uninterruptible wait for write)
> > > 	write directory entry in destination
> > > 	(uninterruptible wait for write)
> > > 	write something else
> > > 
> > > If we freeze some task in place "A)", we'll write to the disk when the
> > > directory write is finished :-(.
> > 
> > Renaming is a single syscall, so won't the process get frozen when the syscall 
> > finishes? 
> 
> It would be frozen when syscall finishes. But if we freeze it at A)
> point, we have a problem.

With this patch, we don't freeze it. :-)

We do something like "send a freeze request to this task, but don't care if it
doesn't freeze".

Now, IMHO, the correctness here is the question of for how long a task can be
in TASK_UNINTERRUPTIBLE it it's not stuck (eg. the resource needed is gone
indefinitely, like in NFS) and I think that's not very long.

For example, if the tasks sleeps on a mutex, then it waits for another task to
release the mutex (interrupt handlers don't use mutexes) and I don't know of
any realistic scenario in which it can last in that state throughout
freeze_processes().

Greetings,
Rafael


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