Le Tue, Mar 18, 2025 at 10:21:48AM -0700, Paul E. McKenney a écrit : > On Fri, Mar 14, 2025 at 03:36:41PM +0100, Frederic Weisbecker wrote: > > It is not possible to send an IPI to a dying CPU that has passed the > > CPUHP_TEARDOWN_CPU stage. Remaining unhandled IPIs are handled later at > > CPUHP_AP_SMPCFD_DYING stage by stop machine. This is the last > > opportunity for RCU exp handler to request an expedited quiescent state. > > And the upcoming final context switch between stop machine and idle must > > have reported the requested context switch. > > > > Therefore, it should not be possible to observe a pending requested > > expedited quiescent state when RCU finally stops watching the outgoing > > CPU. Once IPIs aren't possible anymore, the QS for the target CPU will > > be reported on its behalf by the RCU exp kworker. > > > > Provide an assertion to verify those expectations. > > > > Signed-off-by: Frederic Weisbecker <frede...@kernel.org> > > But what do we do if this assertion triggers?
It means there is likely something to fix because an IPI has been sent and somehow the CPU missed it. > And do we want it to take > effect only in kernels built with CONFIG_PROVE_RCU? Or is such a broken > assumption bad enough to justify a splat in production kernels? > > If the answer to the last question is "yes" (and you, not me, work for > a distro, so it is your question to answer): I think it's bad enough to deserve a real warning. Also this is a slow path. > > Reviewed-by: Paul E. McKenney <paul...@kernel.org> Thanks! > > Thanx, Paul > > > --- > > kernel/rcu/tree.c | 6 ++++++ > > 1 file changed, 6 insertions(+) > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > index 3fe68057d8b4..79dced5fb72e 100644 > > --- a/kernel/rcu/tree.c > > +++ b/kernel/rcu/tree.c > > @@ -4321,6 +4321,12 @@ void rcutree_report_cpu_dead(void) > > * may introduce a new READ-side while it is actually off the QS masks. > > */ > > lockdep_assert_irqs_disabled(); > > + /* > > + * CPUHP_AP_SMPCFD_DYING was the last call for rcu_exp_handler() > > execution. > > + * The requested QS must have been reported on the last context switch > > + * from stop machine to idle. > > + */ > > + WARN_ON_ONCE(rdp->cpu_no_qs.b.exp); > > // Do any dangling deferred wakeups. > > do_nocb_deferred_wakeup(rdp); > > > > -- > > 2.48.1 > >
