[34-longterm 085/247] sched: suppress RCU lockdep splat in task_fork_fair

Paul Gortmaker
2011-06-23T18:18:38Z

From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

-------------------
This is a commit scheduled for the next v2.6.34 longterm release.
If you see a problem with using this for longterm, please comment.
-------------------

commit b0a0f667a349247bd7f05f806b662a25653822bc upstream.

> ===================================================
> [ INFO: suspicious rcu_dereference_check() usage. ]
> ---------------------------------------------------
> /home/greearb/git/linux.wireless-testing/kernel/sched.c:618 invoked rcu_dereference_check() without protection!
>
> other info that might help us debug this:
>
> rcu_scheduler_active = 1, debug_locks = 1
> 1 lock held by ifup/23517:
> #0: (&rq->lock){-.-.-.}, at: [<c042f782>] task_fork_fair+0x3b/0x108
>
> stack backtrace:
> Pid: 23517, comm: ifup Not tainted 2.6.36-rc6-wl+ #5
> Call Trace:
> [<c075e219>] ? printk+0xf/0x16
> [<c0455842>] lockdep_rcu_dereference+0x74/0x7d
> [<c0426854>] task_group+0x6d/0x79
> [<c042686e>] set_task_rq+0xe/0x57
> [<c042f79e>] task_fork_fair+0x57/0x108
> [<c042e965>] sched_fork+0x82/0xf9
> [<c04334b3>] copy_process+0x569/0xe8e
> [<c0433ef0>] do_fork+0x118/0x262
> [<c076302f>] ? do_page_fault+0x16a/0x2cf
> [<c044b80c>] ? up_read+0x16/0x2a
> [<c04085ae>] sys_clone+0x1b/0x20
> [<c04030a5>] ptregs_clone+0x15/0x30
> [<c0402f1c>] ? sysenter_do_call+0x12/0x38

Here a newly created task is having its runqueue assigned. The new task
is not yet on the tasklist, so cannot go away. This is therefore a false
positive, suppress with an RCU read-side critical section.

Reported-by: Ben Greear <greearb@candelatech.com
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Ben Greear <greearb@candelatech.com
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
---
kernel/sched_fair.c | 5 ++++-
1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 3a0f7ba..b0f68035 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -3592,8 +3592,11 @@ static void task_fork_fair(struct task_struct *p)

update_rq_clock(rq);

- if (unlikely(task_cpu(p) != this_cpu))
+ if (unlikely(task_cpu(p) != this_cpu)) {
+ rcu_read_lock();
__set_task_cpu(p, this_cpu);
+ rcu_read_unlock();
+ }

update_curr(cfs_rq);

--
1.7.4.4