From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755372AbbFRLI0 (ORCPT ); Thu, 18 Jun 2015 07:08:26 -0400 Received: from mail-la0-f53.google.com ([209.85.215.53]:35780 "EHLO mail-la0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753811AbbFRLIU (ORCPT ); Thu, 18 Jun 2015 07:08:20 -0400 MIME-Version: 1.0 In-Reply-To: <20150618095719.GA4528@gmail.com> References: <1d95640676a92a5ff7382e9c87517c12ea23ccd9.1434485184.git.luto@kernel.org> <20150617094114.GA3940@gmail.com> <20150618095719.GA4528@gmail.com> From: Andy Lutomirski Date: Thu, 18 Jun 2015 04:07:58 -0700 Message-ID: Subject: Re: [RFC/INCOMPLETE 01/13] context_tracking: Add context_tracking_assert_state To: Ingo Molnar Cc: Andy Lutomirski , "Paul E. McKenney" , X86 ML , "linux-kernel@vger.kernel.org" , =?UTF-8?B?RnLDqWTDqXJpYyBXZWlzYmVja2Vy?= , Rik van Riel , Oleg Nesterov , Denys Vlasenko , Borislav Petkov , Kees Cook , Brian Gerst Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 18, 2015 at 2:57 AM, Ingo Molnar wrote: > > * Andy Lutomirski wrote: > >> On Wed, Jun 17, 2015 at 2:41 AM, Ingo Molnar wrote: >> > >> > * Andy Lutomirski wrote: >> > >> >> This will let us sprinkle sanity checks around the kernel without >> >> making too much of a mess. >> >> >> >> Signed-off-by: Andy Lutomirski >> >> --- >> >> 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. --Andy