Messages in this thread | | | Date | Thu, 18 Jun 2015 11:57:20 +0200 | From | Ingo Molnar <> | Subject | Re: [RFC/INCOMPLETE 01/13] context_tracking: Add context_tracking_assert_state |
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* 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:
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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