On Tue, May 26, 2015 at 10:13:02PM -0700, Nicholas A. Bellinger wrote: > On Tue, 2015-05-26 at 14:44 +0200, Bart Van Assche wrote: > > On 05/26/15 08:57, Nicholas A. Bellinger wrote: > > > - Add various rcu_dereference and lockless_dereference RCU notation > > > > Hello Nic, > > > > Feedback from an RCU expert (which I'm not) would be appreciated here. > > But my understanding is that lockless_dereference(p) should be used for > > a pointer p that has *not* been annotated as an RCU pointer. I think in > > the for-next branch of the target repository that this macro is used to > > access RCU-annotated pointers. Is that why sparse complains about how > > lockless_dereference() is used in the target tree ? > > > > Was curious about this myself.. Thanks for raising the question! > > The intention of lockless_dereference() in both this and preceding > series is for __rcu protected pointers that are accessed outside of > rcu_read_lock() protection, and who's lifetime is controlled by a: > > - struct kref > - struct percpu_ref > - struct config_group symlink > - RCU updater path with some manner of mutex or spinlock held > > This is supposed to be following Paul's comment in rcupdate.h: > > * Similar to rcu_dereference(), but for situations where the pointed-to > * object's lifetime is managed by something other than RCU. That > * "something other" might be reference counting or simple immortality. > > Paul, would you be to kind to clarify the intention for us..? The lockless_dereference() primitive is to be used for pointers that are -not- marked with __rcu. In fact, the sparse tool should yell at you if you use lockless_dereference() on an __rcu-marked pointer. You could use smp_store_release() to update the pointer when inserting new data. If you are using one of the lists, then the _rcu variant of the list-insert macro should be used (list_add_rcu()), because that is needed to make sure that the reader sees a properly initialized new element. If you have a pointer that is sometimes protected by RCU and other times protected by something else, you still use one of the rcu_dereference() macros to access it. For example, if a given RCU-protected pointer is protected either by RCU or by some lock, you might write common code that is called from either context as follows: p = rcu_dereference_check(pointer, lockdep_is_held(&some_lock)); Does that help, or am I missing your point? Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
