On Tue, Jun 28, 2022 at 12:31:12AM +0000, Zhang, Qiang1 wrote: > > > > Add Cc > > > > Currently, the sync_sched_exp_online_cleanup() is invoked in > > cpuhp per-cpu kthreads when CPU is going online, so the CPU id > > obtained by get_cpu() should always be equal to the CPU id of > > the passed parameter, that is to say, the smp_call_function_single() > > never be invoked, if be invoked, there may be problem with cpu-hotplug, > > this commit add WARN_ON_ONCE() to remind everyone. > > > > Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx> > > --- > > kernel/rcu/tree_exp.h | 2 ++ > > 1 file changed, 2 insertions(+) > > > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > > index be667583a554..ae8dcfd4486c 100644 > > --- a/kernel/rcu/tree_exp.h > > +++ b/kernel/rcu/tree_exp.h > > @@ -865,6 +865,8 @@ static void sync_sched_exp_online_cleanup(int cpu) > > put_cpu(); > > return; > > } > > + > > + WARN_ON_ONCE(my_cpu != cpu); > > >If we are going to add this sort of warning, why not instead add it > >to rcutree_online_cpu()? > > > >The reason the warning has not been present is the prospect of concurrent > >onlining at boot time, which might have rcutree_online_cpu() invoked > >from CPU 0 for multiple CPUs at boot. However, the for_each_online_cpu() > >loop has recently been removed from rcu_init(). > > The rcutree_online_cpu() is always invoked in per-cpu cpuhp kthreads, > and even if it is called directly through the for_each_online_cpu() loop in > rcu_init(), since the rcu_init() takes precedence over smp initialization, > only the CPU0 is online at this time, still only called on CPU0. And sync_sched_exp_online_cleanup() is only ever invoked from rcutree_online_cpu(), correct? Thanx, Paul > Thanks > Zqiang > > > > >But I would like to hear what others think. Would such a warning > >significantly help debugging? > > > > Thanx, Paul > > > > /* Quiescent state needed on some other CPU, send IPI. */ > > ret = smp_call_function_single(cpu, rcu_exp_handler, NULL, 0); > > put_cpu(); > > -- > > 2.25.1 > >
