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

[Thomas Gleixner]

On Wed, Sep 15 2021 at 11:14, Dmitry Vyukov wrote:
> On Wed, 15 Sept 2021 at 10:57, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>> That made me actually look at that mac80211_hwsim callback again.
>>
>>         hrtimer_forward(&data->beacon_timer, hrtimer_get_expires(timer),
>>                         ns_to_ktime(bcn_int * NSEC_PER_USEC));
>>
>> So what this does is really wrong because it tries to schedule the timer
>> on the theoretical periodic timeline. Which goes really south once the
>> timer is late or the callback execution took longer than the
>> period. Hypervisors scheduling out a VCPU at the wrong place will do
>> that for you nicely.
>
> Nice!
>
> You mentioned that hrtimer_run_queues() may not return. Does it mean
> that it can just loop executing the same re-armed callback again and
> again? Maybe then the debug check condition should be that
> hrtimer_run_queues() runs the same callback more than N times w/o
> returning?

Something like that.