On 11/06/2019 18:10, Jacob Pan wrote: >> The issue is theoretical at the moment because no users do this, but >> I'd be more comfortable taking the xa_lock, which prevents a >> concurrent xa_erase()+free(). (I commented on your v3 but you might >> have missed it) >> > Did you reply to my v3? I did not see it. I only saw your comments about > v3 in your commit message.
My fault, I sneaked the comments in a random reply three levels down the thread: https://lore.kernel.org/linux-iommu/836caf0d-699e-33ba-5303-b1c9c949c...@arm.com/ (Great, linux-iommu is indexed by lore! I won't have to Cc lkml anymore) >>>> + ioasid_data = xa_load(&ioasid_xa, ioasid); >>>> + if (ioasid_data) >>>> + rcu_assign_pointer(ioasid_data->private, data); >>> it is good to publish and have barrier here. But I just wonder even >>> for weakly ordered machine, this pointer update is quite far away >>> from its data update. >> >> I don't know, it could be right before calling ioasid_set_data(): >> >> mydata = kzalloc(sizeof(*mydata)); >> mydata->ops = &my_ops; (1) >> ioasid_set_data(ioasid, mydata); >> ... /* no write barrier here */ >> data->private = mydata; (2) >> >> And then another thread calls ioasid_find(): >> >> mydata = ioasid_find(ioasid); >> if (mydata) >> mydata->ops->do_something(); >> >> On a weakly ordered machine, this thread could observe the pointer >> assignment (2) before the ops assignment (1), and dereference NULL. >> Using rcu_assign_pointer() should fix that >> > I agree it is better to have the barrier. Just thought there is already > a rcu_read_lock() in xa_load() in between. rcu_read_lock() may have > barrier in some case but better not count on it. Yes, and even if rcu_read_lock() provided a barrier I don't think it would be sufficient, because acquire semantics don't guarantee that prior writes appear to happen before the barrier, only the other way round. A lock operation with release semantics, for example spin_unlock(), should work. Thanks, Jean > No issues here. I will > integrate this in the next version. > >> Thanks, >> Jean > > [Jacob Pan] >
