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

[Peter Zijlstra <peterz@infradead.org>]

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@infradead.org> 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.

> Would you say expedited is bad enough that we should avoid it here?
> The function is called only when kernel modules are loaded or
> unloaded, so not very frequently.

Module unload is pretty crap (it has stop_machine), so an expedited
would not really be noticable, but module load isn't nearly as bad.

Also, getting the vendor to fix their driver seems like a good thing :-)

So please consider using regular synchronize_rcu() here.