Le Wed, Feb 26, 2025 at 10:26:34AM -0500, Joel Fernandes a écrit : > > > On 2/26/2025 10:04 AM, Paul E. McKenney wrote: > >>> I was wondering if you could also point to the fastpath that this is racing > >>> with, it is not immediately clear (to me) what this smp_mb() is pairing with > >>> 🙁 > >> It is supposed to pair with the barrier in sync_exp_work_done() but then again > >> this is already enforced by the smp_mb__after_unlock_lock() chained through > >> rnp locking. > > You could interpret that "Order GP completion with preceding accesses" > > to include preceding readers, which to your point sounds plausible. > > And in that case, again as you say, the raw_spin_lock_irqsave_rcu_node() > > in rcu_report_exp_rnp() provides the needed ordering. > > > > I think. 😉 > > This is for the case where readers are blocked. For the case where readers were > not blocked, and we exited the RSCS, we just expect the regular QS reporting > paths to call into rcu_report_exp_rnp() and similarly provide the full memory > barrier (smp_mb) on the now-reader-unlocked-CPU right? Right, again through rnp locking and smp_mb__after_unlock_lock(). > > Just wanted to check my understanding was correct :) > > Also if I may paraphrase the ordering requirement here, we do not want RCU > readers to observe any modifications happening to data after the GP has ended > (i.e. synchronize_rcu_expedited() has returned). Similarly, we do not want > updates in the pre-existing readers to not be visible to accesses after the GP > has ended. Right? Exactly! Thanks. > > thanks, > > - Joel
