Messages in this thread Patch in this message | | | Date | Wed, 28 May 2008 21:38:52 -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 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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