linux-kernel - Re: [RFC/INCOMPLETE 01/13] context_tracking: Add context_tracking_assert

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Date:	Wed, 17 Jun 2015 07:15:59 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Andy Lutomirski <luto@...nel.org>,
	"Paul E. McKenney" <paulmck@...ibm.com>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Borislav Petkov <bp@...en8.de>,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: [RFC/INCOMPLETE 01/13] context_tracking: Add context_tracking_assert_state

On Wed, Jun 17, 2015 at 2:41 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Andy Lutomirski <luto@...nel.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@...nel.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.
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().

The assert macro avoids all these problems despite being a bit ugly.
It's either a no-op returning void or it does the right thing.

--Andy
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