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Date: Fri, 15 May 2020 00:30:23 +0200 From: Frederic Weisbecker <frederic@...nel.org> To: "Paul E. McKenney" <paulmck@...nel.org> Cc: LKML <linux-kernel@...r.kernel.org>, Steven Rostedt <rostedt@...dmis.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Lai Jiangshan <jiangshanlai@...il.com>, Joel Fernandes <joel@...lfernandes.org>, Josh Triplett <josh@...htriplett.org> Subject: Re: [PATCH 08/10] rcu: Allow to deactivate nocb on a CPU On Thu, May 14, 2020 at 08:47:07AM -0700, Paul E. McKenney wrote: > On Thu, May 14, 2020 at 12:45:26AM +0200, Frederic Weisbecker wrote: > This last seems best to me. The transition from CBLIST_NOT_OFFLOADED > to CBLIST_OFFLOADING of course needs to be on the CPU in question with > at least bh disabled. Probably best to be holding rcu_nocb_lock(), > but that might just be me being overly paranoid. So that's in the case of offloading, right? Well, I don't think we'd need to even disable bh nor lock nocb. We just need the current CPU to see the local update of cblist->offloaded = CBLIST_OFFLOADING before the kthread is unparked: cblist->offloaded = CBLIST_OFFLOADING; /* Make sure subsequent softirq lock nocb */ barrier(); kthread_unpark(rdp->nocb_cb_thread); Now, although that guarantees that nocb_cb will see CBLIST_OFFLOADING upon unparking, it's not guaranteed that the nocb_gp will see it on its next round. Ok so eventually you're right, I should indeed lock nocb... > > > > > +static long rcu_nocb_rdp_deoffload(void *arg) > > > > +{ > > > > + struct rcu_data *rdp = arg; > > > > + > > > > + WARN_ON_ONCE(rdp->cpu != raw_smp_processor_id()); > > > > + __rcu_nocb_rdp_deoffload(rdp); > > > > + > > > > + return 0; > > > > +} > > > > > > For example, is the problem caused by invocations of this > > > rcu_nocb_rdp_deoffload() function? > > > > How so? > > It looked to me like it wasn't excluding either rcu_barrier() or CPU > hotplug. It might also not have been pinning onto the CPU in question, > but that might just be me misremembering. Then again, I didn't see a > call to it, so maybe its callers set things up appropriately. > > OK, I will bite... What is the purpose of rcu_nocb_rdp_deoffload()? ;-) Ah it's called using work_on_cpu() which launch a workqueue on the target and waits for completion. And that whole thing is protected inside the barrier mutex and hotplug. > Agreed! And I do believe that concurrent callback execution will > prove better than a possibly indefinite gap in callback execution. Mutual agreement! :-) Thanks.
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