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

[Paul E. McKenney]

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?

							Thanx, Paul