Hi Frederic,
On 10/4/2022 6:28 PM, Frederic Weisbecker wrote:
> On Tue, Oct 04, 2022 at 02:41:47AM +0000, Joel Fernandes (Google) wrote:
>> From: Frederic Weisbecker <frederic@xxxxxxxxxx>
>>
>> In preparation of RCU lazy changes, wake up the RCU nocb gp thread if
>
> It's more than just prep work for a new feature, it's a regression fix.
Oh ok, both our fixes are equivalent but I chose yours since its cleaner. I was
fixing Lazy CBs since I can actually trigger this issue.
>> needed after an entrain. Otherwise, the RCU barrier callback can wait in
>> the queue for several seconds before the lazy callbacks in front of it
>> are serviced.
>
> It's not about lazy callbacks here (but you can mention the fact that
> waking nocb_gp if necessary after flushing bypass is a beneficial side
> effect for further lazy implementation).
>
> So here is the possible bad scenario:
>
> 1) CPU 0 is nocb, it queues a callback
> 2) CPU 0 goes idle (or userspace with nohz_full) forever
> 3) The grace period related to that callback elapses
> 4) The callback is moved to the done list (but is not invoked yet), there are no more pending for CPU 0
> 5) CPU 1 calls rcu_barrier() and entrains to CPU 0 cblist
CPU 1 can only entrain into CPU 0 if the CPU is offline:
if (!rcu_rdp_cpu_online(rdp)) {
rcu_barrier_entrain(rdp);
WARN_ON_ONCE(READ_ONCE(rdp->barrier_seq_snap) != gseq);
raw_spin_unlock_irqrestore(&rcu_state.barrier_lock,
...
continue;
}
Otherwise an IPI does the entraining. So I do not see how CPU 0 being idle
causes the cross-CPU entraining.
> 6) CPU 1 waits forever
But, I agree it can still wait forever, once the IPI handler does the
entraining, since nothing will do the GP thread wakeup.
>>
>> Reported-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
>
> Fixes: 5d6742b37727 ("rcu/nocb: Use rcu_segcblist for no-CBs CPUs")
So, do you mind writing a proper patch with a proper commit message and Fixes
tag then? It can independent of this series and add my Reported-by tag, thanks!
Thanks!
- Joel
