Re: [PATCH v2 01/17] add support for Clang CFI

[Sami Tolvanen]

On Fri, Mar 19, 2021 at 6:52 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> On Fri, Mar 19, 2021 at 01:26:59PM +0100, Peter Zijlstra wrote:
> > On Thu, Mar 18, 2021 at 04:48:43PM -0700, Sami Tolvanen wrote:
> > > On Thu, Mar 18, 2021 at 3:29 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > > >
> > > > On Thu, Mar 18, 2021 at 10:10:55AM -0700, Sami Tolvanen wrote:
> > > > > +static void update_shadow(struct module *mod, unsigned long base_addr,
> > > > > +             update_shadow_fn fn)
> > > > > +{
> > > > > +     struct cfi_shadow *prev;
> > > > > +     struct cfi_shadow *next;
> > > > > +     unsigned long min_addr, max_addr;
> > > > > +
> > > > > +     next = vmalloc(SHADOW_SIZE);
> > > > > +
> > > > > +     mutex_lock(&shadow_update_lock);
> > > > > +     prev = rcu_dereference_protected(cfi_shadow,
> > > > > +                                      mutex_is_locked(&shadow_update_lock));
> > > > > +
> > > > > +     if (next) {
> > > > > +             next->base = base_addr >> PAGE_SHIFT;
> > > > > +             prepare_next_shadow(prev, next);
> > > > > +
> > > > > +             min_addr = (unsigned long)mod->core_layout.base;
> > > > > +             max_addr = min_addr + mod->core_layout.text_size;
> > > > > +             fn(next, mod, min_addr & PAGE_MASK, max_addr & PAGE_MASK);
> > > > > +
> > > > > +             set_memory_ro((unsigned long)next, SHADOW_PAGES);
> > > > > +     }
> > > > > +
> > > > > +     rcu_assign_pointer(cfi_shadow, next);
> > > > > +     mutex_unlock(&shadow_update_lock);
> > > > > +     synchronize_rcu_expedited();
> > > >
> > > > expedited is BAD(tm), why is it required and why doesn't it have a
> > > > comment?
> > >
> > > Ah, this uses synchronize_rcu_expedited() because we have a case where
> > > synchronize_rcu() hangs here with a specific SoC family after the
> > > vendor's cpu_pm driver powers down CPU cores.
> >
> > Broken vendor drivers seem like an exceedingly poor reason for this.
>
> The vendor is supposed to make sure that RCU sees the CPU cores as either
> deep idle or offline before powering them down.  My guess is that the
> CPU is powered down, but RCU (and probably much else in the system)
> thinks that the CPU is still up and running.  So I bet that you are
> seeing other issues as well.
>
> I take it that the IPIs from synchronize_rcu_expedited() have the effect
> of momentarily powering up those CPUs?

I suspect you're correct. I'll change this to use synchronize_rcu() in v3.

Sami