linux-kernel - Re: [PATCH tip/core/rcu 53/55] rcu: Warn when srcu_read_lock() is used in an extended quiescent state

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Message-ID: <20111004234237.GA15836@somewhere.redhat.com>
Date:	Wed, 5 Oct 2011 01:42:39 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, patches@...aro.org
Subject: Re: [PATCH tip/core/rcu 53/55] rcu: Warn when srcu_read_lock() is
 used in an extended quiescent state

On Tue, Oct 04, 2011 at 04:40:28PM -0700, Paul E. McKenney wrote:
> On Tue, Oct 04, 2011 at 11:03:29PM +0200, Frederic Weisbecker wrote:
> > On Tue, Sep 06, 2011 at 11:00:47AM -0700, Paul E. McKenney wrote:
> > > Catch SRCU up to the other variants of RCU by making PROVE_RCU
> > > complain if either srcu_read_lock() or srcu_read_lock_held() are
> > > used from within dyntick-idle mode.
> > > 
> > > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > > ---
> > >  include/linux/srcu.h |   25 +++++++++++++++----------
> > >  1 files changed, 15 insertions(+), 10 deletions(-)
> > > 
> > > diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> > > index 58971e8..fcbaee7 100644
> > > --- a/include/linux/srcu.h
> > > +++ b/include/linux/srcu.h
> > > @@ -28,6 +28,7 @@
> > >  #define _LINUX_SRCU_H
> > >  
> > >  #include <linux/mutex.h>
> > > +#include <linux/rcupdate.h>
> > >  
> > >  struct srcu_struct_array {
> > >  	int c[2];
> > > @@ -60,18 +61,10 @@ int __init_srcu_struct(struct srcu_struct *sp, const char *name,
> > >  	__init_srcu_struct((sp), #sp, &__srcu_key); \
> > >  })
> > >  
> > > -# define srcu_read_acquire(sp) \
> > > -		lock_acquire(&(sp)->dep_map, 0, 0, 2, 1, NULL, _THIS_IP_)
> > > -# define srcu_read_release(sp) \
> > > -		lock_release(&(sp)->dep_map, 1, _THIS_IP_)
> > > -
> > >  #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
> > >  
> > >  int init_srcu_struct(struct srcu_struct *sp);
> > >  
> > > -# define srcu_read_acquire(sp)  do { } while (0)
> > > -# define srcu_read_release(sp)  do { } while (0)
> > > -
> > >  #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
> > >  
> > >  void cleanup_srcu_struct(struct srcu_struct *sp);
> > > @@ -90,11 +83,23 @@ long srcu_batches_completed(struct srcu_struct *sp);
> > >   * read-side critical section.  In absence of CONFIG_DEBUG_LOCK_ALLOC,
> > >   * this assumes we are in an SRCU read-side critical section unless it can
> > >   * prove otherwise.
> > > + *
> > > + * Note that if the CPU is in an extended quiescent state, for example,
> > > + * if the CPU is in dyntick-idle mode, then rcu_read_lock_held() returns
> > > + * false even if the CPU did an rcu_read_lock().  The reason for this is
> > > + * that RCU ignores CPUs that are in extended quiescent states, so such
> > > + * a CPU is effectively never in an RCU read-side critical section
> > > + * regardless of what RCU primitives it invokes.  This state of affairs
> > > + * is required -- RCU would otherwise need to periodically wake up
> > > + * dyntick-idle CPUs, which would defeat the whole purpose of dyntick-idle
> > > + * mode.
> > >   */
> > >  static inline int srcu_read_lock_held(struct srcu_struct *sp)
> > >  {
> > >  	if (debug_locks)
> > >  		return lock_is_held(&sp->dep_map);
> > > +	if (rcu_check_extended_qs())
> > > +		return 0;
> > 
> > Just to warn you, While rebasing this, I'm also moving things around:
> 
> Thank you for letting me know, should not be a problem.
> 
> > 	if (!debug_lock)
> > 		return 1;
> > 
> > 	if (rcu_is_cpu_idle())
> > 		return 0;
> > 
> > 	return lock_is_held(&sp->dep_map);
> > 
> > Otherwise we only do the check if lock debugging is disabled,
> > which is not what we want I think.
> 
> Would it make sense to use this order?
> 
> 	if (rcu_is_cpu_idle())
> 		return 0;
> 
> 	if (!debug_lock)
> 		return 1;
> 
> 	return lock_is_held(&sp->dep_map);
> 
> Given the new approach, rcu_is_cpu_idle() works whether or not debug_lock
> is enabled.

Yeah why not.

I'm taking that approach.

Thanks.
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