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Date:	Sat, 1 May 2010 22:00:43 -0400
From:	Miles Lane <miles.lane@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Eric Paris <eparis@...hat.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] RCU: don't turn off lockdep when find suspicious 
	rcu_dereference_check() usage

On Sat, May 1, 2010 at 5:55 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> 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@...ux.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@...hat.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@...hat.com>
>> >> > > Signed-off-by: Lai Jiangshan <laijs@...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@...hat.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
>

What is the rationale for defaulting to showing only one RCU splat?
That setting seems likely to reduce the rate at which things get
cleaned up.
Ciao,
      Miles
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