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

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Message-ID: <20150617094114.GA3940@gmail.com>
Date:	Wed, 17 Jun 2015 11:41:14 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andy Lutomirski <luto@...nel.org>,
	"Paul E. McKenney" <paulmck@...ibm.com>
Cc:	x86@...nel.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


* 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.)

Thanks,

	Ingo
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