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

[Frederic Weisbecker]

On Thu, Jun 18, 2015 at 06:17:29PM +0200, Ingo Molnar wrote:
> 
> * Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> 
> > On Thu, Jun 18, 2015 at 4:07 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> > > On Thu, Jun 18, 2015 at 2:57 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> > >>
> > >> * Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> > >>
> > >>> On Wed, Jun 17, 2015 at 2:41 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> > >>> >
> > >>> > * Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> > >>> >
> > >>> >> This will let us sprinkle sanity checks around the kernel without
> > >>> >> making too much of a mess.
> > >>> >>
> > >>> >> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> > >>> >> ---
> > >>> >>  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.

That sounds good. With good layout of these things, the compiler should still be
able to nop related code when context tracking is disabled.

> 
> Thanks,
> 
> 	Ingo
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