Date:	Wed, 28 May 2008 21:38:52 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Alexey Dobriyan <adobriyan@...il.com>, torvalds@...l.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: 2.6.26-rc4: RIP __call_for_each_cic+0x20/0x50

On Wed, May 28, 2008 at 06:20:12AM -0700, Paul E. McKenney wrote:
> On Wed, May 28, 2008 at 02:44:24PM +0200, Jens Axboe wrote:
> > On Wed, May 28 2008, Paul E. McKenney wrote:
> > > On Wed, May 28, 2008 at 12:07:21PM +0200, Jens Axboe wrote:
> > > > On Tue, May 27 2008, Paul E. McKenney wrote:
> > > > > On Tue, May 27, 2008 at 03:35:10PM +0200, Jens Axboe wrote:
> > > > > > On Tue, May 27 2008, Alexey Dobriyan wrote:
> > > > > > > On Sat, May 10, 2008 at 02:37:19PM +0400, Alexey Dobriyan wrote:
> > > > > > > > > > > > @@ -41,8 +41,8 @@ int put_io_context(struct io_context *ioc)
> > > > > > > > > > > >  		rcu_read_lock();
> > > > > > > > > > > >  		if (ioc->aic && ioc->aic->dtor)
> > > > > > > > > > > >  			ioc->aic->dtor(ioc->aic);
> > > > > > > > > > > > -		rcu_read_unlock();
> > > > > > > > > > > >  		cfq_dtor(ioc);
> > > > > > > > > > > > +		rcu_read_unlock();
> > > > > > > > > > > >  
> > > > > > > > > > > >  		kmem_cache_free(iocontext_cachep, ioc);
> > > > > > > > > > > >  		return 1;
> > > > > > > > > > > 
> > > > > > > > > > > This helps in sense that 3 times bulk cross-compiles finish to the end.
> > > > > > > > > > > You'll hear me if another such oops will resurface.
> > > > > > > > > > 
> > > > > > > > > > Still looking good?
> > > > > > > > > 
> > > > > > > > > Yup!
> > > > > > > > 
> > > > > > > > And this with patch in mainline, again with PREEMPT_RCU.
> > > > > > > 
> > > > > > > Ping, this happened again with 2.6.26-rc4 and PREEMPT_RCU.
> > > > > > 
> > > > > > Worrisome... Paul, would you mind taking a quick look at cfq
> > > > > > and see if you can detect why breaks with preempt rcu? It's
> > > > > > clearly a use-after-free symptom, but I don't see how it can
> > > > > > happen.
> > > > > 
> > > > > Some quick and probably off-the-mark questions...
> > > > 
> > > > Thanks!
> > > 
> > > Glad it actually was of help!  ;-)
> > 
> > Your reviews are ALWAYS greatly appreciated!
> 
> :-)
> 
> > > > > o	What is the purpose of __call_for_each_cic()?  When called
> > > > > 	from call_for_each_cic(), it is under rcu_read_lock(), as
> > > > > 	required, but it is also called from cfq_free_io_context(),
> > > > > 	which is assigned to the ->dtor and ->exit members of the
> > > > > 	cfq_io_context struct.  What protects calls through these
> > > > > 	members?
> > > > > 
> > > > > 	(This is for the ->cic_list field of the cfq_io_context structure.
> > > > > 	One possibility is that the io_context's ->lock member is held,
> > > > > 	but I don't see this.  Not that I looked all that hard...)
> > > > > 
> > > > > 	My suggestion would be to simply change all invocations of
> > > > > 	__call_for_each_cic() to instead invoke call_for_each_cic().
> > > > > 	The rcu_read_lock()/rcu_read_unlock() pair is pretty
> > > > > 	lightweight, even in CONFIG_PREEMPT_RCU.
> > > > 
> > > > __call_for_each_cic() is always called under rcu_read_lock(), it merely
> > > > exists to avoid a double rcu_read_lock(). Even if it is cheap. The
> > > > convention follows the usual __lock_is_already_held() double under
> > > > score, but I guess it could do with a comment! There are only two
> > > > callers of the function, call_for_each_cic() which does the
> > > > rcu_read_lock(), and cfq_free_io_context() which is called from ->dtor
> > > > (and holds the rcu_read_lock() and ->trim which actually does not. That
> > > > looks like it could be problematic, but it's only called when an io
> > > > scheduler module is removed so not really critical. I'll add it, though!
> > > > Actually, the task_lock() should be enough there. So no bug, but (again)
> > > > it could do with a comment.
> > > 
> > > Sounds good!
> > > 
> > > > > o	When calling cfq_slab_kill(), for example from cfq_exit(),
> > > > > 	what ensures that all previous RCU callbacks have completed?
> > > > > 	
> > > > > 	I suspect that you need an rcu_barrier() at the beginning
> > > > > 	of cfq_slab_kill(), but I could be missing something.
> > > > 
> > > > So we have two callers of that, one is from the error path at init time
> > > > and is obviously ok. The other does need rcu_barrier()! I'll add that.
> > > 
> > > OK, that does make my brain hurt less.  ;-)
> > 
> > So that one was also OK, as Fabio pointed out. If you follow the
> > ioc_gone and user tracking, the:
> > 
> >         if (elv_ioc_count_read(ioc_count))
> >                 wait_for_completion(ioc_gone);
> > 
> > also has the side effect of waiting for RCU callbacks calling
> > kmem_cache_free() to have finished as well.
> 
> I stand corrected.

But one additional question...

	static void cfq_cic_free_rcu(struct rcu_head *head)
	{
		struct cfq_io_context *cic;

		cic = container_of(head, struct cfq_io_context, rcu_head);

		kmem_cache_free(cfq_ioc_pool, cic);
		elv_ioc_count_dec(ioc_count);

		if (ioc_gone && !elv_ioc_count_read(ioc_count))
			complete(ioc_gone);
	}

Suppose that a pair of tasks both execute the elv_ioc_count_dec()
at the same time, so that all counters are now zero.  Both then
find that there is still an ioc_gone, and that the count is
now zero.  One of the tasks invokes complete(ioc_gone).  This
awakens the corresponding cfq_exit(), which now returns, getting
rid of its stack frame -- and corrupting the all_gone auto variable
that ioc_gone references.

Now the second task gets a big surprise when it tries to invoke
complete(ioc_gone).

Or is there something else that I am missing here?

							Thanx, Paul

> > > > > o	What protects the first rcu_dereference() in cfq_cic_lookup()?
> > > > > 	There needs to be either an enclose rcu_read_lock() on the
> > > > > 	one hand or the ->queue_lock needs to be held.
> > > > > 
> > > > > 	(My guess is the latter, given the later rcu_assign_pointer()
> > > > > 	in this same function, but I don't see a lock acquisition
> > > > > 	in the immediate vicinity -- might be further up the function
> > > > > 	call stack, though.)
> > > > 
> > > > There's no locking going into that function when coming from
> > > > cfq_get_io_context(), the other paths (happen) to hold the queue lock
> > > > already though.
> > > 
> > > So the call from cfq_get_io_context() needs an rcu_read_lock()?
> > > Not seeing this in the patch below, but maybe you have it up a
> > > function-call level or two?
> > 
> > It's in there, it now does:
> > 
> >         rcu_read_lock();
> >         cic = rcu_dereference(ioc->ioc_data);
> >         if (cic && cic->key == cfqd) {
> >                 rcu_read_unlock();
> >                 return cic;
> >         }
> >         ...
> > 
> > OK? Which is basically what remains of the patch now, except for the
> > comment additions. Oh, and the ioc->lock protecting setting of
> > ->ioc_data as well. New version below. Alexey, care to give this a
> > spin? Seems your box is very well suited for finding RCU preempt
> > problems :-)
> 
> OK, looks good.
> 
> 						Thanx, Paul
> 
> > diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> > index 4df3f05..d01b411 100644
> > --- a/block/cfq-iosched.c
> > +++ b/block/cfq-iosched.c
> > @@ -1142,6 +1142,9 @@ static void cfq_put_queue(struct cfq_queue *cfqq)
> >  	kmem_cache_free(cfq_pool, cfqq);
> >  }
> > 
> > +/*
> > + * Must always be called with the rcu_read_lock() held
> > + */
> >  static void
> >  __call_for_each_cic(struct io_context *ioc,
> >  		    void (*func)(struct io_context *, struct cfq_io_context *))
> > @@ -1197,6 +1200,11 @@ static void cic_free_func(struct io_context *ioc, struct cfq_io_context *cic)
> >  	cfq_cic_free(cic);
> >  }
> > 
> > +/*
> > + * Must be called with rcu_read_lock() held or preemption otherwise disabled.
> > + * Only two callers of this - ->dtor() which is called with the rcu_read_lock(),
> > + * and ->trim() which is called with the task lock held
> > + */
> >  static void cfq_free_io_context(struct io_context *ioc)
> >  {
> >  	/*
> > @@ -1502,20 +1510,24 @@ static struct cfq_io_context *
> >  cfq_cic_lookup(struct cfq_data *cfqd, struct io_context *ioc)
> >  {
> >  	struct cfq_io_context *cic;
> > +	unsigned long flags;
> >  	void *k;
> > 
> >  	if (unlikely(!ioc))
> >  		return NULL;
> > 
> > +	rcu_read_lock();
> > +
> >  	/*
> >  	 * we maintain a last-hit cache, to avoid browsing over the tree
> >  	 */
> >  	cic = rcu_dereference(ioc->ioc_data);
> > -	if (cic && cic->key == cfqd)
> > +	if (cic && cic->key == cfqd) {
> > +		rcu_read_unlock();
> >  		return cic;
> > +	}
> > 
> >  	do {
> > -		rcu_read_lock();
> >  		cic = radix_tree_lookup(&ioc->radix_root, (unsigned long) cfqd);
> >  		rcu_read_unlock();
> >  		if (!cic)
> > @@ -1524,10 +1536,13 @@ cfq_cic_lookup(struct cfq_data *cfqd, struct io_context *ioc)
> >  		k = cic->key;
> >  		if (unlikely(!k)) {
> >  			cfq_drop_dead_cic(cfqd, ioc, cic);
> > +			rcu_read_lock();
> >  			continue;
> >  		}
> > 
> > +		spin_lock_irqsave(&ioc->lock, flags);
> >  		rcu_assign_pointer(ioc->ioc_data, cic);
> > +		spin_unlock_irqrestore(&ioc->lock, flags);
> >  		break;
> >  	} while (1);
> > 
> > @@ -2134,6 +2149,10 @@ static void *cfq_init_queue(struct request_queue *q)
> > 
> >  static void cfq_slab_kill(void)
> >  {
> > +	/*
> > +	 * Caller already ensured that pending RCU callbacks are completed,
> > +	 * so we should have no busy allocations at this point.
> > +	 */
> >  	if (cfq_pool)
> >  		kmem_cache_destroy(cfq_pool);
> >  	if (cfq_ioc_pool)
> > @@ -2292,6 +2311,11 @@ static void __exit cfq_exit(void)
> >  	ioc_gone = &all_gone;
> >  	/* ioc_gone's update must be visible before reading ioc_count */
> >  	smp_wmb();
> > +
> > +	/*
> > +	 * this also protects us from entering cfq_slab_kill() with
> > +	 * pending RCU callbacks
> > +	 */
> >  	if (elv_ioc_count_read(ioc_count))
> >  		wait_for_completion(ioc_gone);
> >  	cfq_slab_kill();
> > 
> > -- 
> > Jens Axboe
> > 
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