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>

diff --git a/kernel/sched.c b/kernel/sched.c
index 2e3545f57e770830c4fca6702a64e3e2bca510a1..deb5ac8c12f37c44e71dcc46484149d073430948 100644 (file)
--- 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;