On Tue, Dec 20, 2022 at 6:05 PM Frederic Weisbecker <frederic@xxxxxxxxxx> wrote: > > On Tue, Dec 20, 2022 at 07:06:57PM +0000, Joel Fernandes wrote: > > On Tue, Dec 20, 2022 at 7:01 PM Mathieu Desnoyers > > <mathieu.desnoyers@xxxxxxxxxxxx> wrote: > > > > > > On 2022-12-20 13:29, Joel Fernandes wrote: > > > > > > > > > > > I do want to finish my memory barrier studies of SRCU over the holidays since I have been deep in the hole with that already. Back to the post flip memory barrier here since I think now even that might not be needed… > > > > > > I strongly suspect the memory barrier after flip is useless for the same > > > reasons I mentioned explaining why the barrier before the flip is useless. > > > > > > However, we need to double-check that we have memory barriers at the > > > beginning and end of synchronize_srcu, and between load of "unlock" > > > counters and load of "lock" counters. > > > > > > Where is the barrier at the beginning of synchronize_srcu ? > > > > I believe we don't need another memory barrier at the beginning of > > synchronize_srcu() (but this part of my SRCU study is still pending > > ;)) . The grace period guarantee (read-side critical sections don't > > span the GP) is already enforced by the memory barrier between > > scanning for all unlocks, and scanning for all locks (Paired with > > corresponding memory barriers on the read-side). > > > > Accordingly, before we scan all locks and match lock == unlock, there > > is an smp_mb(). Why is that not sufficient? > > That's not enough, you still need a barrier between the updater's pre-GP > accesses and the scans, so that post-GP read side sees the updater's pre-GP > accesses: > > > UPDATER READER > ------- ------ > WRITE A WRITE srcu_read_lock > smp_mb() //rcu_seq_snap() smp_mb() > READ srcu_read_lock //scans READ A But see the comments also in srcu_readers_active_idx_check() * Needs to be a smp_mb() as the read side may * contain a read from a variable that is written to before the * synchronize_srcu() in the write side So that appears to be already covered. Or is your point that the scans are not happening on the same CPU as the pre-GP writer, as scans are happening from workqueue ? Perhaps that comment misled me. Confused, - Joel
