Re: [RFC 6/6] RCU: Track rcu_dereference() in RCU read-side critical section

[Josh Triplett <josh@joshtriplett.org>]

On Tue, Feb 16, 2016 at 11:01:43AM +0800, Boqun Feng wrote:
> On Mon, Feb 15, 2016 at 05:05:53PM -0800, Paul E. McKenney wrote:
> > On Thu, Feb 04, 2016 at 12:45:12AM +0800, Boqun Feng wrote:
> > > The variables protected by an RCU read-side critical section are
> > > sometimes hard to figure out, especially when the critical section is
> > > long or has some function calls in it. However, figuring out which
> > > variable a RCU read-side critical section protects could save
> > > us a lot of time for code reviewing, bug fixing or performance tuning.
> > > 
> > > This patch therefore uses the LOCKED_ACCESS to collect the information
> > > of relationship between rcu_dereference*() and rcu_read_lock*() by
> > > doing:
> > > 
> > > Step 0: define a locked_access_class for RCU.
> > > 
> > > Step 1: set the content of rcu_*_lock_key and __srcu_key to the
> > > address of the locked_access_class for RCU.
> > > 
> > > Step 2: add locked_access_point() in __rcu_dereference_check()
> > > 
> > > After that we can figure out not only in which RCU read-side critical
> > > section but also after which rcu_read_lock*() called an
> > > rcu_dereference*() is called.
> > > 
> > > This feature is controlled by a config option RCU_LOCKED_ACCESS.
> > > 
> > > Also clean up the initialization code of lockdep_maps for different
> > > flavors of RCU a little bit.
> > > 
> > > Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> > > ---
> > >  include/linux/rcupdate.h |  8 ++++++++
> > >  include/linux/srcu.h     |  8 +++++++-
> > >  kernel/locking/lockdep.c |  3 +++
> > >  kernel/rcu/update.c      | 31 +++++++++++++++++--------------
> > >  lib/Kconfig.debug        | 12 ++++++++++++
> > >  5 files changed, 47 insertions(+), 15 deletions(-)
> > > 
> > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > > index 14e6f47..4bab658 100644
> > > --- a/include/linux/rcupdate.h
> > > +++ b/include/linux/rcupdate.h
> > > @@ -610,6 +610,13 @@ static inline void rcu_preempt_sleep_check(void)
> > >  #define rcu_dereference_sparse(p, space)
> > >  #endif /* #else #ifdef __CHECKER__ */
> > > 
> > > +#ifdef CONFIG_RCU_LOCKED_ACCESS
> > > +extern struct locked_access_class rcu_laclass;
> > > +#define rcu_dereference_access() \
> > > +	locked_access_point(&rcu_laclass, LOCKED_ACCESS_TYPE_READ)
> > > +#else /* #ifdef CONFIG_LOCKED_ACCESS */
> > > +#define rcu_dereference_access()
> > > +#endif /* #else #ifdef CONFIG_LOCKED_ACCESS */
> > >  #define __rcu_access_pointer(p, space) \
> > >  ({ \
> > >  	typeof(*p) *_________p1 = (typeof(*p) *__force)READ_ONCE(p); \
> > > @@ -622,6 +629,7 @@ static inline void rcu_preempt_sleep_check(void)
> > >  	typeof(*p) *________p1 = (typeof(*p) *__force)lockless_dereference(p); \
> > >  	RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_check() usage"); \
> > >  	rcu_dereference_sparse(p, space); \
> > > +	rcu_dereference_access(); \
> > >  	((typeof(*p) __force __kernel *)(________p1)); \
> > >  })
> > >  #define __rcu_dereference_protected(p, c, space) \
> > > diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> > > index f5f80c5..ff048a2 100644
> > > --- a/include/linux/srcu.h
> > > +++ b/include/linux/srcu.h
> > > @@ -64,12 +64,18 @@ struct srcu_struct {
> > > 
> > >  #ifdef CONFIG_DEBUG_LOCK_ALLOC
> > > 
> > > +# ifdef CONFIG_RCU_LOCKED_ACCESS
> > > +# define RCU_KEY { .laclass = &rcu_laclass }
> > > +# else
> > > +# define RCU_KEY { 0 }
> > > +# endif
> > > +
> > >  int __init_srcu_struct(struct srcu_struct *sp, const char *name,
> > >  		       struct lock_class_key *key);
> > > 
> > >  #define init_srcu_struct(sp) \
> > >  ({ \
> > > -	static struct lock_class_key __srcu_key; \
> > > +	static struct lock_class_key __srcu_key = RCU_KEY; \
> > 
> > This gets me the following for the TASKS01, TINY02, TREE02, TREE05,
> > TREE06, and TREE08 configurations:
> > 
> >   CC      mm/mmap.o
> > In file included from /home/paulmck/public_git/linux-rcu/include/linux/notifier.h:15:0,
> >                  from /home/paulmck/public_git/linux-rcu/arch/x86/include/asm/kdebug.h:4,
> >                  from /home/paulmck/public_git/linux-rcu/include/linux/kdebug.h:4,
> >                  from /home/paulmck/public_git/linux-rcu/kernel/notifier.c:1:
> > /home/paulmck/public_git/linux-rcu/kernel/notifier.c: In function ‘srcu_init_notifier_head’:
> > /home/paulmck/public_git/linux-rcu/include/linux/srcu.h:78:16: warning: missing braces around initializer [-Wmissing-braces]
> >   static struct lock_class_key __srcu_key = RCU_KEY; \
> >                 ^
> > /home/paulmck/public_git/linux-rcu/kernel/notifier.c:526:6: note: in expansion of macro ‘init_srcu_struct’
> >   if (init_srcu_struct(&nh->srcu) < 0)
> >       ^
> > /home/paulmck/public_git/linux-rcu/include/linux/srcu.h:78:16: warning: (near initialization for ‘__srcu_key.subkeys’) [-Wmissing-braces]
> >   static struct lock_class_key __srcu_key = RCU_KEY; \
> >                 ^
> > /home/paulmck/public_git/linux-rcu/kernel/notifier.c:526:6: note: in expansion of macro ‘init_srcu_struct’
> >   if (init_srcu_struct(&nh->srcu) < 0)
> >       ^
> > 
> > These have the following in their .config files:
> > 
> > 	CONFIG_DEBUG_LOCK_ALLOC=y
> > 
> > However, none of your new Kconfig options are set, which needs to be
> > tested because this will be the common case.  Several of them have
> > CONFIG_PROVE_LOCKING=y, but others do not.
> > 
> 
> Oh.. this is embarrassing ;-(
> 
> Hmm.. when CONFIG_RCU_LOCKED_ACCESS=n and CONFIG_DEBUG_LOCK_ALLOC=y,
> this line becomes:
> 
> 	static struct lock_class_key __srcu_key = { 0 };
> 
> IIUC, "={ 0 }" is the unverisal zero initializer in C, could be used for
> zero initialising any structure or array, so it's OK here, right?

Compilers seem touchy about that.  Sometimes { 0 } works, and sometimes
{} works (that really should *always* work, but it doesn't).

However, for a static struct, you can always just leave off the
initializer.

- Josh Triplett