Re: [PATCH] RCU: don't turn off lockdep when find suspicious rcu_dereference_check() usage

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sat, May 01, 2010 at 01:26:15PM -0400, Miles Lane wrote:
> On Tue, Apr 20, 2010 at 9:52 AM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > On Tue, Apr 20, 2010 at 08:45:28AM -0400, Miles Lane wrote:
> >> Is there a patch set for 2.6.34-rc5 I can test?
> >
> > I will be sending a patchset out later today after testing, but
> > please see below for a sneak preview collapsed into a single patch.
> >
> >                                                        Thanx, Paul
> >
> >> On Tue, Apr 20, 2010 at 8:31 AM, Eric Paris <eparis@redhat.com> wrote:
> >>
> >> > On Tue, 2010-04-20 at 16:23 +0800, Lai Jiangshan wrote:
> >> >
> >> > > [PATCH] RCU: don't turn off lockdep when find suspicious
> >> > rcu_dereference_check() usage
> >> > >
> >> > > When suspicious rcu_dereference_check() usage is detected, lockdep is
> >> > still
> >> > > available actually, so we should not call debug_locks_off() in
> >> > > lockdep_rcu_dereference().
> >> > >
> >> > > For get rid of too much "suspicious rcu_dereference_check() usage"
> >> > > output when the "if(!debug_locks_off())" statement is removed. This patch
> >> > uses
> >> > > static variable '__warned's for very usage of "rcu_dereference*()".
> >> > >
> >> > > One variable per usage, so, Now, we can get multiple complaint
> >> > > when we detect multiple different suspicious rcu_dereference_check()
> >> > usage.
> >> > >
> >> > > Requested-by: Eric Paris <eparis@redhat.com>
> >> > > Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
> >> >
> >> > Although mine was a linux-next kernel and it doesn't appear that I have
> >> > rcu_dereference_protected() at all, so I dropped that bit of the patch,
> >> > it worked great!  I got 4 more complaints to harass people with.  Feel
> >> > free to add my tested by if you care to.
> >> >
> >> > Tested-by: Eric Paris <eparis@redhat.com>
> >
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index 07db2fe..ec9ab49 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -190,6 +190,15 @@ static inline int rcu_read_lock_sched_held(void)
> >
> >  #ifdef CONFIG_PROVE_RCU
> >
> > +#define __do_rcu_dereference_check(c)                                  \
> > +       do {                                                            \
> > +               static bool __warned;                                   \
> > +               if (debug_lockdep_rcu_enabled() && !__warned && !(c)) { \
> > +                       __warned = true;                                \
> > +                       lockdep_rcu_dereference(__FILE__, __LINE__);    \
> > +               }                                                       \
> > +       } while (0)
> > +
> >  /**
> >  * rcu_dereference_check - rcu_dereference with debug checking
> >  * @p: The pointer to read, prior to dereferencing
> > @@ -219,8 +228,7 @@ static inline int rcu_read_lock_sched_held(void)
> >  */
> >  #define rcu_dereference_check(p, c) \
> >        ({ \
> > -               if (debug_lockdep_rcu_enabled() && !(c)) \
> > -                       lockdep_rcu_dereference(__FILE__, __LINE__); \
> > +               __do_rcu_dereference_check(c); \
> >                rcu_dereference_raw(p); \
> >        })
> >
> > @@ -237,8 +245,7 @@ static inline int rcu_read_lock_sched_held(void)
> >  */
> >  #define rcu_dereference_protected(p, c) \
> >        ({ \
> > -               if (debug_lockdep_rcu_enabled() && !(c)) \
> > -                       lockdep_rcu_dereference(__FILE__, __LINE__); \
> > +               __do_rcu_dereference_check(c); \
> >                (p); \
> >        })
> >
> > diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
> > index da5e139..e5c0244 100644
> > --- a/kernel/cgroup_freezer.c
> > +++ b/kernel/cgroup_freezer.c
> > @@ -205,9 +205,12 @@ static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task)
> >         * No lock is needed, since the task isn't on tasklist yet,
> >         * so it can't be moved to another cgroup, which means the
> >         * freezer won't be removed and will be valid during this
> > -        * function call.
> > +        * function call.  Nevertheless, apply RCU read-side critical
> > +        * section to suppress RCU lockdep false positives.
> >         */
> > +       rcu_read_lock();
> >        freezer = task_freezer(task);
> > +       rcu_read_unlock();
> >
> >        /*
> >         * The root cgroup is non-freezable, so we can skip the
> > diff --git a/kernel/lockdep.c b/kernel/lockdep.c
> > index 2594e1c..03dd1fa 100644
> > --- a/kernel/lockdep.c
> > +++ b/kernel/lockdep.c
> > @@ -3801,8 +3801,6 @@ void lockdep_rcu_dereference(const char *file, const int line)
> >  {
> >        struct task_struct *curr = current;
> >
> > -       if (!debug_locks_off())
> > -               return;
> >        printk("\n===================================================\n");
> >        printk(  "[ INFO: suspicious rcu_dereference_check() usage. ]\n");
> >        printk(  "---------------------------------------------------\n");
> > diff --git a/kernel/sched.c b/kernel/sched.c
> > index 6af210a..14c44ec 100644
> > --- a/kernel/sched.c
> > +++ b/kernel/sched.c
> > @@ -323,6 +323,15 @@ static inline struct task_group *task_group(struct task_struct *p)
> >  /* Change a task's cfs_rq and parent entity if it moves across CPUs/groups */
> >  static inline void set_task_rq(struct task_struct *p, unsigned int cpu)
> >  {
> > +       /*
> > +        * Strictly speaking this rcu_read_lock() is not needed since the
> > +        * task_group is tied to the cgroup, which in turn can never go away
> > +        * as long as there are tasks attached to it.
> > +        *
> > +        * However since task_group() uses task_subsys_state() which is an
> > +        * rcu_dereference() user, this quiets CONFIG_PROVE_RCU.
> > +        */
> > +       rcu_read_lock();
> >  #ifdef CONFIG_FAIR_GROUP_SCHED
> >        p->se.cfs_rq = task_group(p)->cfs_rq[cpu];
> >        p->se.parent = task_group(p)->se[cpu];
> > @@ -332,6 +341,7 @@ static inline void set_task_rq(struct task_struct *p, unsigned int cpu)
> >        p->rt.rt_rq  = task_group(p)->rt_rq[cpu];
> >        p->rt.parent = task_group(p)->rt_se[cpu];
> >  #endif
> > +       rcu_read_unlock();
> >  }
> >
> >  #else
> >
> 
> Hi Paul.
> 
> Has this patch made it into the Linus tree?
> Thanks!

Hello, Miles,

Not yet -- working with Ingo to get a variant of it into -tip on
its way to Linus's tree.  The latest patch stack may be found at
http://lkml.org/lkml/2010/4/30/500.

						Thanx, Paul