sched: fix possible recursive rq->lock

Vaidyanathan Srinivasan reported:

 > =============================================
 > [ INFO: possible recursive locking detected ]
 > 2.6.28-autotest-tip-sv #1
 > ---------------------------------------------
 > klogd/5062 is trying to acquire lock:
 >  (&rq->lock){++..}, at: [<ffffffff8022aca2>] task_rq_lock+0x45/0x7e
 >
 > but task is already holding lock:
 >  (&rq->lock){++..}, at: [<ffffffff805f7354>] schedule+0x158/0xa31

With sched_mc at 2. (it is default-off)

Strictly speaking we'll not deadlock, because ttwu will not be able to
place the migration task on our rq, but since the code can deal with
both rqs getting unlocked, this seems the easiest way out.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 5 insertions, 0 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 2e3545f57e77..deb5ac8c12f3 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3728,8 +3728,13 @@ redo:
                 }
 
                 double_unlock_balance(this_rq, busiest);
+                /*
+                 * Should not call ttwu while holding a rq->lock
+                 */
+                spin_unlock(&this_rq->lock);
                 if (active_balance)
                         wake_up_process(busiest->migration_thread);
+                spin_lock(&this_rq->lock);
 
         } else
                 sd->nr_balance_failed = 0;