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    SubjectRe: 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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