Messages in this thread | | | Date | Wed, 28 May 2008 03:30:26 -0700 | From | "Paul E. McKenney" <> | Subject | Re: 2.6.26-rc4: RIP __call_for_each_cic+0x20/0x50 |
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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! ;-)
> > 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. ;-)
> > o Updates to the ->ioc_data field of the cfq_io_context > > seem to be protected by the request_queue ->queue_lock > > field. This seems very strange to me. It is OK if every > > cfq_io_context is associated with only one request_queue > > structure -- is this the case? > > ->ioc_data is part of the io_context, not cfq_io_context. And it can be > shared now, so the correct locking for that would be ioc->lock and not > the queue lock. __cfq_exit_single_io_context() is serialized in the > sense that only one process gets to call the exit path.
Makes sense to me!
> > 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?
> > o Why is there no grace period associated with the ioc_data? > > For example, what happens to the old value of ->ioc_data > > after the rcu_assign_pointer() in cfq_cic_lookup()? Readers > > might still be referencing the old version, right? If so, > > how do we avoid messing them up? > > > > Or are we somehow leveraging the call_rcu() in cfq_cic_free()? > > The data belonging to ->ioc_data (the cic, or per-process per-queue > context) is only freed through call_rcu().
Ah, OK, got it.
> > Any of this at all helpful? > > Very, perhaps with a few more rounds we can find some more bugs :-). I'm > attaching a patch below, how does that look?
Looks much improved! Very interested to hear how it does with the testing.
Thanx, Paul
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c > index 4df3f05..75db529 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 +2149,11 @@ static void *cfq_init_queue(struct request_queue *q) > > static void cfq_slab_kill(void) > { > + /* > + * Make sure that all existing RCU callbacks have been processed > + */ > + rcu_barrier(); > + > if (cfq_pool) > kmem_cache_destroy(cfq_pool); > if (cfq_ioc_pool) > > -- > Jens Axboe >
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