On 28/07/21 01:08, Frederic Weisbecker wrote:
> On Wed, Jul 21, 2021 at 12:51:17PM +0100, Valentin Schneider wrote:
>> Signed-off-by: Valentin Schneider <valentin.schneider@xxxxxxx>
>> ---
>> kernel/rcu/tree_plugin.h | 3 +--
>> 1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
>> index ad0156b86937..6c3c4100da83 100644
>> --- a/kernel/rcu/tree_plugin.h
>> +++ b/kernel/rcu/tree_plugin.h
>> @@ -70,8 +70,7 @@ static bool rcu_rdp_is_offloaded(struct rcu_data *rdp)
>> !(lockdep_is_held(&rcu_state.barrier_mutex) ||
>> (IS_ENABLED(CONFIG_HOTPLUG_CPU) && lockdep_is_cpus_held()) ||
>> rcu_lockdep_is_held_nocb(rdp) ||
>> - (rdp == this_cpu_ptr(&rcu_data) &&
>> - !(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible())) ||
>> + (rdp == this_cpu_ptr(&rcu_data) && is_pcpu_safe()) ||
>
> I fear that won't work. We really need any caller of rcu_rdp_is_offloaded()
> on the local rdp to have preemption disabled and not just migration disabled,
> because we must protect against concurrent offloaded state changes.
>
> The offloaded state is changed by a workqueue that executes on the target rdp.
>
> Here is a practical example where it matters:
>
> CPU 0
> -----
> // =======> task rcuc running
> rcu_core {
> rcu_nocb_lock_irqsave(rdp, flags) {
> if (!rcu_segcblist_is_offloaded(rdp->cblist)) {
> // is not offloaded right now, so it's going
> // to just disable IRQs. Oh no wait:
> // preemption
> // ========> workqueue running
> rcu_nocb_rdp_offload();
> // ========> task rcuc resume
> local_irq_disable();
> }
> }
> ....
> rcu_nocb_unlock_irqrestore(rdp, flags) {
> if (rcu_segcblist_is_offloaded(rdp->cblist)) {
> // is offloaded right now so:
> raw_spin_unlock_irqrestore(rdp, flags);
>
> And that will explode because that's an impaired unlock on nocb_lock.
Harumph, that doesn't look good, thanks for pointing this out.
AFAICT PREEMPT_RT doesn't actually require to disable softirqs here (since
it forces RCU callbacks on the RCU kthreads), but disabled softirqs seem to
be a requirement for much of the underlying functions and even some of the
callbacks (delayed_put_task_struct() ~> vfree() pays close attention to
in_interrupt() for instance).
Now, if the offloaded state was (properly) protected by a local_lock, do
you reckon we could then keep preemption enabled?
>From a naive outsider PoV, rdp->nocb_lock looks like a decent candidate,
but it's a *raw* spinlock (I can't tell right now whether changing this is
a horrible idea or not), and then there's
81c0b3d724f4 ("rcu/nocb: Avoid ->nocb_lock capture by corresponding CPU")
on top...
