Messages in this thread | | | Date | Wed, 28 May 2008 06:20:12 -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 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.
> > > > 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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