On Sat, Feb 12, 2022 at 12:14:20AM +0530, Mukesh Ojha wrote: > > On 2/10/2022 3:36 AM, Paul E. McKenney wrote: > > On Wed, Feb 09, 2022 at 11:53:33PM +0530, Mukesh Ojha wrote: > > > On 2/5/2022 4:25 AM, Paul E. McKenney wrote: > > > > Although it is usually safe to invoke synchronize_rcu_expedited() from a > > > > preemption-enabled CPU-hotplug notifier, if it is invoked from a notifier > > > > between CPUHP_AP_RCUTREE_ONLINE and CPUHP_AP_ACTIVE, its attempts to > > > > invoke a workqueue handler will hang due to RCU waiting on a CPU that > > > > the scheduler is not paying attention to. This commit therefore expands > > > > use of the existing workqueue-independent synchronize_rcu_expedited() > > > > from early boot to also include CPUs that are being hotplugged. > > > > > > > > Link: https://lore.kernel.org/lkml/7359f994-8aaf-3cea-f5cf-c0d3929689d6@xxxxxxxxxxx/ > > > > Reported-by: Mukesh Ojha <quic_mojha@xxxxxxxxxxx> > > > > Cc: Tejun Heo <tj@xxxxxxxxxx> > > > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > > > > --- > > > > kernel/rcu/tree_exp.h | 14 ++++++++++---- > > > > 1 file changed, 10 insertions(+), 4 deletions(-) > > > > > > > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > > > > index 60197ea24ceb9..1a45667402260 100644 > > > > --- a/kernel/rcu/tree_exp.h > > > > +++ b/kernel/rcu/tree_exp.h > > > > @@ -816,7 +816,7 @@ static int rcu_print_task_exp_stall(struct rcu_node *rnp) > > > > */ > > > > void synchronize_rcu_expedited(void) > > > > { > > > > - bool boottime = (rcu_scheduler_active == RCU_SCHEDULER_INIT); > > > > + bool no_wq; > > > > struct rcu_exp_work rew; > > > > struct rcu_node *rnp; > > > > unsigned long s; > > > > @@ -841,9 +841,15 @@ void synchronize_rcu_expedited(void) > > > > if (exp_funnel_lock(s)) > > > > return; /* Someone else did our work for us. */ > > > > + /* Don't use workqueue during boot or from an incoming CPU. */ > > > > + preempt_disable(); > > > > + no_wq = rcu_scheduler_active == RCU_SCHEDULER_INIT || > > > > + !cpumask_test_cpu(smp_processor_id(), cpu_active_mask); > > > > + preempt_enable(); > > > > + > > > > /* Ensure that load happens before action based on it. */ > > > > - if (unlikely(boottime)) { > > > > - /* Direct call during scheduler init and early_initcalls(). */ > > > > + if (unlikely(no_wq)) { > > > > + /* Direct call for scheduler init, early_initcall()s, and incoming CPUs. */ > > > > rcu_exp_sel_wait_wake(s); > > > > } else { > > > > /* Marshall arguments & schedule the expedited grace period. */ > > > > @@ -861,7 +867,7 @@ void synchronize_rcu_expedited(void) > > > > /* Let the next expedited grace period start. */ > > > > mutex_unlock(&rcu_state.exp_mutex); > > > > - if (likely(!boottime)) > > > > + if (likely(!no_wq)) > > > > destroy_work_on_stack(&rew.rew_work); > > > > } > > > > EXPORT_SYMBOL_GPL(synchronize_rcu_expedited); > > > Can we reach a condition after this change where no_wq = true and during > > > rcu_stall report where exp_task = 0 list and exp_mask contain only this cpu > > > ? > > Hello, Mukesh, and thank you for looking this over! > > > > At first glance, I do not believe that this can happen because the > > expedited grace-period machinery avoids waiting on the current CPU. > > (See sync_rcu_exp_select_node_cpus(), both the raw_smp_processor_id() > > early in the function and the get_cpu() later in the function.) > > > > But please let me know if I am missing something here. > > > > But suppose that we could in fact reach this condition. What bad thing > > would happen? Other than a resched_cpu() having been invoked several > > times on a not-yet-online CPU, of course. ;-) > > > I thought more about this, what if synchronize_rcu_expedited thread got > schedule out and run on some other cpu > and we clear out cpu on which it ran next from exp_mask. > > Queuing the work on same cpu ensures that it will always be right cpu to > clear out. > Do you think this can happen ? Indeed it might. But if it did, the scheduler would invoke RCU's hook, which is named rcu_note_context_switch(), and do so on the pre-switch CPU. There are two implementations for this function, one for CONFIG_PREEMPT=y and another for CONFIG_PREEMPT=n. Both look to me like they invoke rcu_report_exp_rdp() when needed, one directly and the other via the CONFIG_PREEMPT=n variant of rcu_qs(). Am I missing something? Thanx, Paul