On Thu, 20 Feb 2025 at 23:36, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: [...] > Suppose that one function walks an RCU-protected list, calling some > function from some other subsystem on each element. Suppose that each > element has another RCU protected list. > > It would be good if the two subsystems could just choose their desired > flavor of RCU reader, without having to know about each other. That's what I figured might be the case - thanks for clarifying. > > Another problem was that if we want to indicate that "RCU" read lock > > is held, then we should just be able to write > > "__must_hold_shared(RCU)", and it shouldn't matter if rcu_read_lock() > > or rcu_read_lock_bh() was used. Previously each of them acquired their > > own capability "RCU" and "RCU_BH" respectively. But rather, we're > > dealing with one acquiring a superset of the other, and expressing > > that is also what I attempted to solve. > > Let me rethink this... > > Would it work to have just one sort of RCU reader, relying on a separate > BH-disable capability for the additional semantics of rcu_read_lock_bh()? That's what I've tried with this patch (rcu_read_lock_bh() also acquires "RCU", on top of "RCU_BH"). I need to add a re-entrancy test, and make sure it doesn't complain about that. At a later stage we might also want to add more general "BH" and "IRQ" capabilities to denote they're disabled when held, but that'd overcomplicate the first version of this series. Thanks, -- Marco
