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? 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):
Reviewed-by: Paul E. McKenney <paul...@kernel.org>
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
>
