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

[Andy Lutomirski]

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.

--Andy
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