Re: [syzbot] INFO: rcu detected stall in syscall_exit_to_user_mode

[Thomas Gleixner]

On Tue, Sep 14 2021 at 11:31, Paul E. McKenney wrote:
> On Tue, Sep 14, 2021 at 08:00:04PM +0200, Dmitry Vyukov wrote:
>> If I understand it correctly the timer is not actually set up as
>> periodic, but rather each callback invocation arms it again. Setting
>> up a timer for 1 ns _once_ (or few times) is probably fine (right?),
>> so the check needs to be somewhat more elaborate and detect "infinite"
>> rearming.
>
> If it were practical, I would suggest checking for a CPU never actually
> executing any instructions in the interrupted context.  The old-school
> way of doing this was to check the amount of time spent interrupted,
> perhaps adding some guess at interrupt entry/exit overhead.  Is there
> a better new-school way?

Set NR_CPUS=0 and if then any executed instruction is observed the bug
is pretty obvious, isn't it?