Re: [RFC/INCOMPLETE 01/13] context_tracking: Add context_tracking_assert_state

[Ingo Molnar <mingo@kernel.org>]

* Andy Lutomirski <luto@amacapital.net> wrote:

> On Thu, Jun 18, 2015 at 4:07 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> > On Thu, Jun 18, 2015 at 2:57 AM, Ingo Molnar <mingo@kernel.org> wrote:
> >>
> >> * Andy Lutomirski <luto@amacapital.net> wrote:
> >>
> >>> On Wed, Jun 17, 2015 at 2:41 AM, Ingo Molnar <mingo@kernel.org> wrote:
> >>> >
> >>> > * Andy Lutomirski <luto@kernel.org> wrote:
> >>> >
> >>> >> This will let us sprinkle sanity checks around the kernel without
> >>> >> making too much of a mess.
> >>> >>
> >>> >> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> >>> >> ---
> >>> >>  include/linux/context_tracking.h | 8 ++++++++
> >>> >>  1 file changed, 8 insertions(+)
> >>> >>
> >>> >> diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
> >>> >> index 2821838256b4..0fbea4b152e1 100644
> >>> >> --- a/include/linux/context_tracking.h
> >>> >> +++ b/include/linux/context_tracking.h
> >>> >> @@ -57,6 +57,13 @@ static inline void context_tracking_task_switch(struct task_struct *prev,
> >>> >>       if (context_tracking_is_enabled())
> >>> >>               __context_tracking_task_switch(prev, next);
> >>> >>  }
> >>> >> +
> >>> >> +static inline void context_tracking_assert_state(enum ctx_state state)
> >>> >> +{
> >>> >> +     rcu_lockdep_assert(!context_tracking_is_enabled() ||
> >>> >> +                        this_cpu_read(context_tracking.state) == state,
> >>> >> +                        "context tracking state was wrong");
> >>> >> +}
> >>> >
> >>> > Please don't introduce assert() style debug check interfaces!
> >>> >
> >>> > (And RCU should be fixed too I suspect.)
> >>> >
> >>> > They are absolutely horrible on the brain when mixed with WARN_ON() interfaces,
> >>> > which are the dominant runtime check interface in the kernel.
> >>> >
> >>> > Instead make it something like:
> >>> >
> >>> >   #define ct_state() (this_cpu_read(context_tracking.state))
> >>> >
> >>> >   #define CT_WARN_ON(cond) \
> >>> >         WARN_ON(context_tracking_is_enabled() && (cond))
> >>> >
> >>> > and then the debug checks can be written as:
> >>> >
> >>> >         CT_WARN_ON(ct_state() != CONTEXT_KERNEL);
> >>> >
> >>> > This is IMHO _far_ more readable than:
> >>> >
> >>> >         context_tracking_assert_state(CONTEXT_KERNEL);
> >>> >
> >>> > ok?
> >>> >
> >>> > (Assuming people will accept 'ct/CT' as an abbreviation for context tracking.)
> >>>
> >>> Hmm, ok I guess.  The part I don't like is having ct_state() at all on
> >>> non-context-tracking kernels -- it seems like it's asking for trouble.
> >>
> >> Well:
> >>
> >>  - if # CONFIG_CONTEXT_TRACKING is not se, then CT_WARN_ON() does nothing.
> >>
> >>  - if CONFIG_CONTEXT_TRACKING=y, but !context_tracking_is_enabled(), then
> >>    CT_WARN_ON() will evaluate 'cond', but won't calculate it.
> >>
> >>  - only if CONFIG_CONTEXT_TRACKING=y && context_tracking_is_enabled() should we
> >>    get as far as ct_state() evaluation.
> >>
> >> so I'm not sure I see the problem you are seeing.
> >>
> >>> We could make CT_WARN_ON not even evaluate its argument if
> >>> !CONFIG_CONTEXT_TRACKING, but then we still have ct_state() returning garbage if
> >>> !context_tracking_is_enabled().
> >>
> >> My understanding is that if !context_tracking_is_enabled() then the compiler
> >> should not even try to evaluate the rest. This is why doing a NULL pointer check
> >> like this is safe:
> >
> > I'm fine with everything you just covered.  My only objection is that,
> > if ct_state() exists, then someone might call it outside CT_WARN_ON,
> > in which case it will break on non-context-tracking setups.
> 
> The more I think about it, the more I dislike ct_state().  We have
> in_atomic(), which is already problematic because the return value
> isn't reliable.  ct_state(), if callable on non context-tracking
> kernels, will also be unreliable.  I prefer things like
> lockdep_assert_held because they can't be misused.
> 
> It would be far too easy for someone to read:
> 
> CT_WARN_ON(ct_state() != CONTEXT_KERNEL);
> 
> and add:
> 
> if (ct_state() == CONTEXT_KERNEL)
>   do_something();
> 
> and that would be bad.

But ct_state() could be made reliable: if !context_tracking_is_enabled() then it 
should return -1 or so.

I.e. we could make it something like:

        enum ctx_state {
                CONTEXT_DISABLED	= -1,
                CONTEXT_KERNEL		=  0,
                CONTEXT_USER		=  1,
                CONTEXT_GUEST		=  2,
        } state;

static inline enum ctx_state ct_state(void)
{
	if (context_tracking_is_enabled())
		return this_cpu_read(context_tracking.state))

	return CONTEXT_DISABLED;
}

and then CT_WARN_ON() DTRT.

Thanks,

	Ingo