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Date: Tue, 20 Apr 2010 06:52:27 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Miles Lane <miles.lane@...il.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 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
--
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