On Mon, 2008-01-07 at 20:49 +0300, Oleg Nesterov wrote:
quoted text
> On 01/07, Oleg Nesterov wrote:
> >
> > Consider this "just for illustration" patch,
> > 
> > --- t/kernel/lockdep.c	2007-11-09 12:57:31.000000000 +0300
> > +++ t/kernel/lockdep.c	2008-01-07 19:43:50.000000000 +0300
> > @@ -1266,10 +1266,13 @@ check_deadlock(struct task_struct *curr,
> >  	struct held_lock *prev;
> >  	int i;
> >  
> > -	for (i = 0; i < curr->lockdep_depth; i++) {
> > +	for (i = curr->lockdep_depth; --i >= 0; ) {
> >  		prev = curr->held_locks + i;
> >  		if (prev->class != next->class)
> >  			continue;
> > +
> > +		if (prev->trylock == -1)
> > +			return 2;
> >  		/*
> >  		 * Allow read-after-read recursion of the same
> >  		 * lock class (i.e. read_lock(lock)+read_lock(lock)):
> > -------------------------------------------------------------------------
> > 
> > Now,
> > 
> > 	// trylock == -1
> > 	#define	spin_mark_nested(l)	\
> > 		lock_acquire(&(l)->dep_map, 0, -1, 0, 2, _THIS_IP_)
> > 	#define	spin_unmark_nested(l)	\
> > 		lock_release(&(l)->dep_map, 1, _THIS_IP_)
> > 
> > and ep_poll_safewake() can do:
> > 
> > 	/* Do really wake up now */
> > 	spin_mark_nested(&wq->lock);
> > 	wake_up(wq);
> > 	spin_unmark_nested(&wq->lock);
> 
> I tested the patch above with the following code,
> 
> 	wait_queue_head_t w1, w2, w3;
> 
> 	init_waitqueue_head(&w1);
> 	init_waitqueue_head(&w2);
> 	init_waitqueue_head(&w3);
> 
> 	local_irq_disable();
> 	spin_lock(&w1.lock);
> 
> 	spin_mark_nested(&w2.lock);
> 	spin_lock(&w2.lock);
> 
> 	spin_mark_nested(&w3.lock);
> 	wake_up(&w3);
> 	spin_unmark_nested(&w3.lock);
> 
> 	spin_unlock(&w2.lock);
> 	spin_unmark_nested(&w2.lock);
> 
> 	spin_unlock(&w1.lock);
> 	local_irq_enable();
> 
> seems to work. What do you think?

I've been pondering this for a while, and some days I really like it,
some days I don't.

The problem I have with it is that it becomes very easy to falsely
annotate problems away - its a very powerful annotation. That said, its
almost powerful enough to annotate the device semaphore/mutex problem.

I think I'll do wake_up_nested() for now and keep this around.

Thanks for this very nice idea though.

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