On Thu, Apr 07, 2016 at 11:47:00AM +0200, Petr Mladek wrote:
> On Wed 2016-04-06 11:33:56, Josh Poimboeuf wrote:
> > On Wed, Apr 06, 2016 at 03:06:19PM +0200, Petr Mladek wrote:
> > > On Fri 2016-03-25 14:34:52, Josh Poimboeuf wrote:
> > > > This is a horrible way to detect whether a task has been preempted.
> > > > Come up with something better: task flag? or is there already an
> > > > existing mechanism?
> > >
> > > What about using kallsyms_lookup_size_offset() to check the address.
> > > It is more heavyweight but less hacky. The following code seems
> > > to work for me:
> > >
> > > bool in_preempt_schedule_irq(unsigned long addr)
> > > {
> > > static unsigned long size;
> > >
> > > if (unlikely(!size)) {
> > > int ret;
> > >
> > > ret = kallsyms_lookup_size_offset(
> > > (unsigned long)preempt_schedule_irq,
> > > size, NULL);
> ^^^^
> It works even better with &size ;-)
>
> > >
> > > /*
> > > * Warn when the function is used without kallsyms or
> > > * when it is unable to locate preempt_schedule_irq().
> > > * Be conservative and always return true in this case.
> > > */
> > > if (WARN_ON(!ret))
> > > size = -1L;
> > > }
> > >
> > > return (addr - (unsigned long)preempt_schedule_irq <= size);
> > > }
> >
> > Yeah, that would definitely be better. Though still somewhat hacky.
>
> Yeah. Well this is the same approach that we use to check if a patched
> function is on the stack.
Oh, I agree that it's a good way to check if preempt_schedule_irq() is
on the stack. I'm just not convinced that's the cleanest way to ask
"has this task been preempted".
> We could even move this check into the livepatch code but then
> print_context_stack_reliable() will not always give reliable results.
Why would moving the check to the livepatch code affect the reliability
of print_context_stack_reliable()?
--
Josh
