On Mon, Jul 26, 2021 at 02:49:03PM -0400, Steven Rostedt wrote: > OK. I see the issue you are saying. And this came from my assumption > that the tracepoint code did a synchronization when unregistering the > last callback. But of course it wont because that would make a lot of > back to back synchronizations of a large number of tracepoints being > unregistered at once. > > And doing it for all 0->1 or 1->0 or even a 1->0->1 can be costly. > > One way to handle this is when going from 1->0, set off a worker that > will do the synchronization asynchronously, and if a 0->1 comes in, > have that block until the synchronization is complete. This should > work, and not have too much of an overhead. > > If one 1->0 starts the synchronization, and one or more 1->0 > transitions happen, it will be recorded where the worker will do > another synchronization, to make sure all 1->0 have went through a full > synchronization before a 0->1 can happen. > > If a 0->1 comes in while a synchronization is happening, it will note > the current "number" for the synchronizations (if another one is > queued, it will wait for one more), before it can begin. As locks will > be held while waiting for synchronizations to finish, we don't need to > worry about another 1->0 coming in while a 0->1 is waiting. Wouldn't get_state_synchronize_rcu() and cond_synchronize_rcu() get you what you need?
