* Andy Lutomirski <l...@amacapital.net> wrote:
> On Wed, Jun 17, 2015 at 2:41 AM, Ingo Molnar <mi...@kernel.org> wrote:
> >
> > * Andy Lutomirski <l...@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 <l...@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:
if (tsk && tsk->field) {
...
}
> The assert macro avoids all these problems despite being a bit ugly.
but writing good kernel code is all about not being ugly...
Thanks,
Ingo
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