Re: 2.6.26-rc4: RIP __call_for_each_cic+0x20/0x50

[Jens Axboe <jens.axboe@oracle.com>]

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.

> > > 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 :-)


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