Re: [RCU] zombie task hung in synchronize_rcu_expedited

[Oleg Nesterov <oleg@redhat.com>]

On 06/07, Oleg Nesterov wrote:
>
> On 06/07, Wei Fu wrote:
> >
> > Yes. I applied your patch on v5.15.160 and run reproducer for 5 hours.
> > I didn't see this issue. Currently, it looks good!. I will continue that test
> > on this weekend.
>
> Great, thanks!
>
> > In last reply, you mentioned TIF_NOTIFY_SIGNAL related to busy-wait loop.
> > Would you please explain why flag-clear works here?
>
> Sure, I'll write the changelog with the explanation and send the patch on
> weekend. If it passes your testing.

Please see the patch I've sent. The changelog doesn't bother to describe this
particular problem because busy-waiting can obviously cause multiple problems,
especially without CONFIG_PREEMPT or if rt_task().

So let me add more details about this particular deadlock here.

The sub-namespace init task T spins in a tight loop calling kernel_wait4()
which returns -EINTR without sleeping because its child C has not exited
yet and signal_pending(T) is true due to TIF_NOTIFY_SIGNAL.

The exiting child C sleeps in synchronize_rcu() which hangs exactly because
T never calls schedule/rcu_note_context_switch, it can't be preempted because
CONFIG_PREEMPT is not enabled.

Note also that without PREEMPT_RCU __rcu_read_lock() is just preempt_disable()
which is nop without CONFIG_PREEMPT.

Oleg.