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From: tip-bot for Peter Zijlstra on 30 Apr 2010 15:20
Commit-ID: 8b08ca52f5942c21564bbb90ccfb61053f2c26a1
Gitweb: http://git.kernel.org/tip/8b08ca52f5942c21564bbb90ccfb61053f2c26a1
Author: Peter Zijlstra <peterz(a)infradead.org>
AuthorDate: Wed, 21 Apr 2010 13:02:07 -0700
Committer: Ingo Molnar <mingo(a)elte.hu>
CommitDate: Fri, 30 Apr 2010 12:03:17 +0200

rcu: Fix RCU lockdep splat in set_task_cpu on fork path

Add an RCU read-side critical section to suppress this false
positive.

Located-by: Eric Paris <eparis(a)parisplace.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra(a)chello.nl>
Signed-off-by: Paul E. McKenney <paulmck(a)linux.vnet.ibm.com>
Cc: laijs(a)cn.fujitsu.com
Cc: dipankar(a)in.ibm.com
Cc: mathieu.desnoyers(a)polymtl.ca
Cc: josh(a)joshtriplett.org
Cc: dvhltc(a)us.ibm.com
Cc: niv(a)us.ibm.com
Cc: peterz(a)infradead.org
Cc: rostedt(a)goodmis.org
Cc: Valdis.Kletnieks(a)vt.edu
Cc: dhowells(a)redhat.com
Cc: eric.dumazet(a)gmail.com
LKML-Reference: <1271880131-3951-1-git-send-email-paulmck(a)linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo(a)elte.hu>
---
kernel/sched.c | 10 ++++++++++
1 files changed, 10 insertions(+), 0 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index de0bd26..3c2a54f 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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