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

[Andy Lutomirski]

On Thu, Jun 18, 2015 at 9:26 AM, Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:
> 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.

Done.

--Andy

>
>>
>> Thanks,
>>
>>       Ingo



-- 
Andy Lutomirski
AMA Capital Management, LLC
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