On Thu, Jun 30, 2005 at 09:13:28AM -0700, Sean Hefty wrote: > Libor Michalek wrote: > > Assume that the userspace 'struct ib_cm_event' contains the cm_id as > > well as a new 'u64 context' which is inherited from the cm_id, and is > > set at the time of the cm_id creation. This is what I'm assuming that > > Arlin would like to see. > > > > In the case of two threads accessing the CM at once there's a race > > condition if you are going to use the 'context' variable as a pointer > > to memory: > > > > Thread 1 Thread 2 > > ------------------------------------- ----------------------------------- > > cm_object = malloc(sizeof(*cm_object) > > ib_cm_create_id(&cm_object->cm_id, > > (u64)cm_object) > > > > ib_cm_event_get(&event) > > ib_cm_destroy_id(cm_object->cm_id) > > free(cm_object); > > process_event((void *)event->context); > > I see. This appears to come from a difference between the event reporting > model used by the kernel CM versus the usermode CM (callback versus > calldown).
Do you block the destroy on a lock while a callback for that cm_id is active? I wouldn't say that the difference is attributed to callback vs. calldown, in both cases it's a matter of serializing the destroy with the event. > Maybe there's a way to assist the user here. Can we report a > destruction event, or require a second call to indicate that an event has > been processed? A destruction event could work, but with some limits which might make it impracticle. The user would have to be really carefull not to do _anything_ with the object after calling destroy, and only cleanup in the same thread that is used to get the destroy completion event. The destroy completion event could be retreived and processed before the original destroy call returns. Also, the user would need to make sure that they are getting events in a _single_ thread, since multiple event get threads could pose the same problem as before. Blocking on the destroy seems like it could be error prone, that you could easily deadlock the user, who probably has a lock around the object which contains the cm_id... We could build the serialization table for the API consumer, have all cm_id calls and events go through a level of indirection in a table locked against multiple threads. This was the way we ended up doing it in our old code for the userCM that we used for uDAPL. I had left this out since it seems reasonable that not all apps would want/need this guarantee from the API, and that they could implement it themselves if they did want it... I could be wrong. -Libor _______________________________________________ openib-general mailing list [email protected] http://openib.org/mailman/listinfo/openib-general To unsubscribe, please visit http://openib.org/mailman/listinfo/openib-general
