From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date: Thu, 10 Feb 2011 09:22:08 +0000 (+0100)
Subject: sched: suppress RCU lockdep splat in task_fork_fair
X-Git-Tag: firefly_0821_release~10186^2~479
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=f4de371f2acbfd0516a7123235c428f88da5c85c;p=firefly-linux-kernel-4.4.55.git

sched: suppress RCU lockdep splat in task_fork_fair

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: Mike Galbraith <efault@gmx.de>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
---

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 01e311e6b47f..e9dba101d18b 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1992,8 +1992,11 @@ 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);