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: <20111004234028.GK2223@linux.vnet.ibm.com>
Date:	Tue, 4 Oct 2011 16:40:28 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.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 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.

							Thanx, Paul

> >  	return 1;
> >  }
> >  
> > @@ -150,7 +155,7 @@ static inline int srcu_read_lock(struct srcu_struct *sp) __acquires(sp)
> >  {
> >  	int retval = __srcu_read_lock(sp);
> >  
> > -	srcu_read_acquire(sp);
> > +	rcu_lock_acquire(&(sp)->dep_map);
> >  	return retval;
> >  }
> >  
> > @@ -164,7 +169,7 @@ static inline int srcu_read_lock(struct srcu_struct *sp) __acquires(sp)
> >  static inline void srcu_read_unlock(struct srcu_struct *sp, int idx)
> >  	__releases(sp)
> >  {
> > -	srcu_read_release(sp);
> > +	rcu_lock_release(&(sp)->dep_map);
> >  	__srcu_read_unlock(sp, idx);
> >  }
> >  
> > -- 
> > 1.7.3.2
> > 
> > --
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