From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754264Ab2BDNNk (ORCPT ); Sat, 4 Feb 2012 08:13:40 -0500 Received: from mail-we0-f174.google.com ([74.125.82.174]:62366 "EHLO mail-we0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753355Ab2BDNNj (ORCPT ); Sat, 4 Feb 2012 08:13:39 -0500 Date: Sat, 4 Feb 2012 14:13:35 +0100 From: Frederic Weisbecker To: "Paul E. McKenney" Cc: Heiko Carstens , linux-kernel@vger.kernel.org, Ingo Molnar , Peter Zijlstra Subject: Re: rcu warnings cause stack overflow Message-ID: <20120204131333.GB13382@somewhere> References: <20120201100652.GA2922@osiris.boeblingen.de.ibm.com> <20120201151445.GA6731@somewhere.redhat.com> <20120202122742.GA2745@osiris.boeblingen.de.ibm.com> <20120202145219.GD9071@somewhere.redhat.com> <20120202191116.GF2518@linux.vnet.ibm.com> <20120203093214.GA2471@osiris.boeblingen.de.ibm.com> <20120203183334.GG2382@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120203183334.GG2382@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Feb 03, 2012 at 10:33:35AM -0800, Paul E. McKenney wrote: > On Fri, Feb 03, 2012 at 10:32:14AM +0100, Heiko Carstens wrote: > > On Thu, Feb 02, 2012 at 11:11:16AM -0800, Paul E. McKenney wrote: > > > On Thu, Feb 02, 2012 at 03:52:20PM +0100, Frederic Weisbecker wrote: > > > > On Thu, Feb 02, 2012 at 01:27:42PM +0100, Heiko Carstens wrote: > > > > > On Wed, Feb 01, 2012 at 04:14:48PM +0100, Frederic Weisbecker wrote: > > > > > > > Removing the WARN_ON_ONCE will fix this and, if lockdep is turned on, still > > > > > > > will find illegal uses. But it won't work for lockdep off configs... > > > > > > > So we probably want something better than the patch below. > > > > > > > > > > > > Ah ok. Hmm, but why are you using an exception to implement WARN_ON() > > > > > > in s390? Is it to have a whole new stack for the warning path in order > > > > > > to avoid stack overflow from the place that called the WARN_ON() ? > > > > > > > > > > The reason was to reduce the code footprint of the WARN_ON() and also > > > > > be able to print the register contents at the time the warning happened. > > > > > > > > Ah ok, makes sense. > > > > > > So Frederic should push his anti-recursion patch, then? > > > > Yes, please. > > > > Tested-by: Heiko Carstens > > > > It still generates recursive warnings because the WARNON_ONCE is inlined and > > every different usage will generate an exception, but it didn't produce a > > stack overflow anymore. > > To avoid the recursive warning the patch below would help. Not sure if it's > > worth it... > > > > Subject: [PATCH] rcu: move rcu_is_cpu_idle() check warning into C file > > > > From: Heiko Carstens > > > > rcu_read_lock() and rcu_read_unlock() generate a warning if a cpu is in > > extended quiescant state. Since these functions are inlined this can cause > > a lot of warnings if in the processing of the WARN_ON_ONCE() there is > > another usage of e.g. rcu_read_lock(). To make sure we only get one > > warning (and avoid possible stack overflows) uninline the check. > > > > Signed-off-by: Heiko Carstens > > --- > > include/linux/rcupdate.h | 9 +++++++-- > > kernel/rcupdate.c | 6 ++++++ > > 2 files changed, 13 insertions(+), 2 deletions(-) > > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > > index 81c04f4..9fe7be5 100644 > > --- a/include/linux/rcupdate.h > > +++ b/include/linux/rcupdate.h > > @@ -230,22 +230,27 @@ static inline void destroy_rcu_head_on_stack(struct rcu_head *head) > > > > #ifdef CONFIG_PROVE_RCU > > extern int rcu_is_cpu_idle(void); > > +extern void rcu_warn_if_is_cpu_idle(void); > > #else /* !CONFIG_PROVE_RCU */ > > static inline int rcu_is_cpu_idle(void) > > { > > return 0; > > } > > + > > +static inline void rcu_warn_if_is_cpu_idle(void) > > +{ > > +} > > #endif /* else !CONFIG_PROVE_RCU */ > > > > static inline void rcu_lock_acquire(struct lockdep_map *map) > > { > > - WARN_ON_ONCE(rcu_is_cpu_idle()); > > + rcu_warn_if_is_cpu_idle(); > > Thank you for the patch, but this WARN_ON_ONCE() has now been removed > in favor of lockdep-RCU checks elsewhere. This has the advantage of > leveraging lockdep's splat-once and anti-recursion facilities. > > So I believe that current -rcu covers this. (And yes, I do need to > push my most recent changes out.) This still uncovers cases where we call rcu_read_lock() without matching rcu_dereference(). Amongst this we have rcu_dereference_raw(), conditional rcu_dereference() and may be cases where we simply have no rcu_dereference* but we use rcu_read_lock() alone for some reason...