RE: [PATCH 1/1] PM: Thaws refrigerated and to be exited kernelthreads

[Dasgupta, Romit]

(Resending with 80 column restriction) 

> -----Original Message-----
> From: Pavel Machek [mailto:pavel@xxxxxx] 
> Sent: Sunday, November 08, 2009 1:57 PM
> To: Dasgupta, Romit
> Cc: Rafael J. Wysocki; linux-omap@xxxxxxxxxxxxxxx; 
> linux-kernel@xxxxxxxxxxxxxxx; linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx
> Subject: Re: [PATCH 1/1] PM: Thaws refrigerated and to be 
> exited kernel threads
> 
> On Sun 2009-11-08 09:52:52, Dasgupta, Romit wrote:
> > > Subject: Re: [PATCH 1/1] PM: Thaws refrigerated and to be 
> exited kernel
> > > threads
> > > 
> > > Hi!
> > > 
> > > > Kicks out a frozen thread from the refrigerator when an 
> active thread has
> > > > invoked kthread_stop on the frozen thread.
> ...
> > > > @@ -49,7 +50,7 @@ void refrigerator(void)
> > > >
> > > >  	for (;;) {
> > > >  		set_current_state(TASK_UNINTERRUPTIBLE);
> > > > -		if (!frozen(current))
> > > > +		if (!frozen(current) || (!current->mm 
> && kthread_should_stop()))
> > > >  			break;
> > > >  		schedule();
> > > 
> > > Well, what if the thread does some processing before 
> stopping? That
> > > would break refrigerator assumptions...
> > 
> > The suspend thread will block until the 'to be stopped' 
> thread clears up. That is what any call to kthread_stop would 
> boil down to. The target thread would anyway be out of the 
> refrigerator so I am not sure what assumption you mean here. 
> Eventually, the target thread would clear up and wake up the 
> suspend thread and then things would go on as usual.
> 
> (Please format to 80 columns).
> 
> No, I do not get it.
> 
> Lets say we have
> 
> evil_data_writer thread that needs to be stopped becuase it writes to
> filesystem
> 
> nofreeze random_stopper thread
> 
> now we create the suspend image, and start writing it out. But that's
> okay, evil_data_writer is stopped so it can't do no harm. But now
> random_stopper decides to thread_stop() the evil_data_writer, and this
> new code allows it to exit the refrigerator, *do some writing*, and
> then stop.
> 
> That's bad, right?
evil_data_writer will enter refrigerator after invoking 'try_to_freeze'. This 
should be followed by a call to kthread_should_stop. There it decides if it 
needs to exit the thread (after cleanups if necessary) or not. I have seen that
the bdi_writeback_task function is like that. It does not care if there is 
pending data to be written if it detects that someone have invoked a 
'kthread_stop' on it. It simply exits. I have seen some other kernel threads 
that do not follow this and I think that probably is not right. 
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