Re: [RFC][PATCH 7/8] entry,hrtimer: Push reprogramming timers into the interrupt return path

[Peter Zijlstra]

On Tue, Sep 23, 2025 at 09:52:40AM +0200, Peter Zijlstra wrote:

> > 2) I came up with the following scenario, which is broken with this
> >    delayed rearm.
> > 
> >    Assume this happens on the timekeeping CPU.
> > 
> >       hrtimer_interrupt()
> >         expire_timers();
> >         set(TIF_REARM);
> > 
> >       exit_to_user_mode_prepare()
> >         handle_tif_muck()
> >           ...
> >           to = jiffies + 2;
> >           while (!cond() && time_before(jiffies, to))
> >           	relax();
> > 
> >      If cond() does not become true for whatever reason, then this won't
> >      make progress ever because the tick hrtimer which increments
> >      jiffies is not happening.
> > 
> >      It can also be a wait on a remote CPU preventing progress
> >      indirectly or a subtle dependency on a timer (timer list or
> >      hrtimer) to expire.
> > 
> >   I have no idea whether that's related to the reported 0-day fallout,
> >   but it definitely is a real problem lurking in the dark.
> 
> Argh... that exit_to_user_mode_loop() thing enables IRQs. Yes, buggered
> something mighty.
> 
> Let me haz a poke.

Bah. So schedule() is first in the TIF loop. Delaying hrtimer_rearm()
until that first schedule() call might just be enough, but that also
means running all of sched_submit_work() without timers... it might just
work, but urgh.

Let me try that anyway. I'll push it out to the robot, we'll see what
happens.