From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756587AbbFRQ0o (ORCPT ); Thu, 18 Jun 2015 12:26:44 -0400 Received: from mail-wg0-f45.google.com ([74.125.82.45]:33040 "EHLO mail-wg0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756072AbbFRQ0l (ORCPT ); Thu, 18 Jun 2015 12:26:41 -0400 Date: Thu, 18 Jun 2015 18:26:37 +0200 From: Frederic Weisbecker To: Ingo Molnar Cc: Andy Lutomirski , Andy Lutomirski , "Paul E. McKenney" , X86 ML , "linux-kernel@vger.kernel.org" , Rik van Riel , Oleg Nesterov , Denys Vlasenko , Borislav Petkov , Kees Cook , Brian Gerst Subject: Re: [RFC/INCOMPLETE 01/13] context_tracking: Add context_tracking_assert_state Message-ID: <20150618162636.GB841@lerouge> References: <1d95640676a92a5ff7382e9c87517c12ea23ccd9.1434485184.git.luto@kernel.org> <20150617094114.GA3940@gmail.com> <20150618095719.GA4528@gmail.com> <20150618161729.GB5799@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150618161729.GB5799@gmail.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 18, 2015 at 06:17:29PM +0200, Ingo Molnar wrote: > > * Andy Lutomirski wrote: > > > On Thu, Jun 18, 2015 at 4:07 AM, Andy Lutomirski wrote: > > > 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. > > > > The more I think about it, the more I dislike ct_state(). We have > > in_atomic(), which is already problematic because the return value > > isn't reliable. ct_state(), if callable on non context-tracking > > kernels, will also be unreliable. I prefer things like > > lockdep_assert_held because they can't be misused. > > > > It would be far too easy for someone to read: > > > > CT_WARN_ON(ct_state() != CONTEXT_KERNEL); > > > > and add: > > > > if (ct_state() == CONTEXT_KERNEL) > > do_something(); > > > > and that would be bad. > > But ct_state() could be made reliable: if !context_tracking_is_enabled() then it > should return -1 or so. > > I.e. we could make it something like: > > enum ctx_state { > CONTEXT_DISABLED = -1, > CONTEXT_KERNEL = 0, > CONTEXT_USER = 1, > CONTEXT_GUEST = 2, > } state; > > static inline enum ctx_state ct_state(void) > { > if (context_tracking_is_enabled()) > return this_cpu_read(context_tracking.state)) > > return CONTEXT_DISABLED; > } > > and then CT_WARN_ON() DTRT. That sounds good. With good layout of these things, the compiler should still be able to nop related code when context tracking is disabled. > > Thanks, > > Ingo