Date:	Fri, 3 Feb 2012 10:33:35 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: rcu warnings cause stack overflow

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 <heiko.carstens@...ibm.com>
> 
> 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 <heiko.carstens@...ibm.com>
> 
> 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 <heiko.carstens@...ibm.com>
> ---
>  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.)

							Thanx, Paul

>  	lock_acquire(map, 0, 0, 2, 1, NULL, _THIS_IP_);
>  }
> 
>  static inline void rcu_lock_release(struct lockdep_map *map)
>  {
> -	WARN_ON_ONCE(rcu_is_cpu_idle());
> +	rcu_warn_if_is_cpu_idle();
>  	lock_release(map, 1, _THIS_IP_);
>  }
> 
> diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
> index 2bc4e13..5deca18 100644
> --- a/kernel/rcupdate.c
> +++ b/kernel/rcupdate.c
> @@ -141,6 +141,12 @@ int rcu_my_thread_group_empty(void)
>  	return thread_group_empty(current);
>  }
>  EXPORT_SYMBOL_GPL(rcu_my_thread_group_empty);
> +
> +void rcu_warn_if_is_cpu_idle(void)
> +{
> +	WARN_ON_ONCE(rcu_is_cpu_idle());
> +}
> +EXPORT_SYMBOL_GPL(rcu_warn_if_is_cpu_idle);
>  #endif /* #ifdef CONFIG_PROVE_RCU */
> 
>  #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD

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