On Wed, Nov 04, 2020 at 03:31:35PM +0100, Frederic Weisbecker wrote: [...] > > > > > + rcu_segcblist_offload(cblist, false); > > > + raw_spin_unlock_rcu_node(rnp); > > > + > > > + if (rdp->nocb_cb_sleep) { > > > + rdp->nocb_cb_sleep = false; > > > + wake_cb = true; > > > + } > > > + rcu_nocb_unlock_irqrestore(rdp, flags); > > > + > > > + if (wake_cb) > > > + swake_up_one(&rdp->nocb_cb_wq); > > > + > > > + swait_event_exclusive(rdp->nocb_state_wq, > > > + !rcu_segcblist_test_flags(cblist, > > > SEGCBLIST_KTHREAD_CB)); > > > + > > > + return 0; > > > +} > > > + > > > +static long rcu_nocb_rdp_deoffload(void *arg) > > > +{ > > > + struct rcu_data *rdp = arg; > > > + > > > + WARN_ON_ONCE(rdp->cpu != raw_smp_processor_id()); > > > > I think this warning can actually happen, if I understand how workqueue > > works correctly. Consider that the corresponding cpu gets offlined right > > after the rcu_nocb_cpu_deoffloaed(), and the workqueue of that cpu > > becomes unbound, and IIUC, workqueues don't do migration during > > cpu-offlining, which means the worker can be scheduled to other CPUs, > > and the work gets executed on another cpu. Am I missing something here?. > > We are holding cpus_read_lock() in rcu_nocb_cpu_offload(), this should > prevent from that. >
But what if the work doesn't get executed until we cpus_read_unlock() and someone offlines that CPU? Regards, Boqun > Thanks!