sched: Clarify ordering between task_rq_lock() and move_queued_task()

There was a wee bit of confusion around the exact ordering here;
clarify things.

Reported-by: Kirill Tkhai <ktkhai@parallels.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20150217121258.GM5029@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 16 insertions, 0 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 1f37fe7f77a4..fd0a6ed21849 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -341,6 +341,22 @@ static struct rq *task_rq_lock(struct task_struct *p, unsigned long *flags)
                 raw_spin_lock_irqsave(&p->pi_lock, *flags);
                 rq = task_rq(p);
                 raw_spin_lock(&rq->lock);
+                /*
+                 *      move_queued_task()              task_rq_lock()
+                 *
+                 *      ACQUIRE (rq->lock)
+                 *      [S] ->on_rq = MIGRATING         [L] rq = task_rq()
+                 *      WMB (__set_task_cpu())          ACQUIRE (rq->lock);
+                 *      [S] ->cpu = new_cpu             [L] task_rq()
+                 *                                      [L] ->on_rq
+                 *      RELEASE (rq->lock)
+                 *
+                 * If we observe the old cpu in task_rq_lock, the acquire of
+                 * the old rq->lock will fully serialize against the stores.
+                 *
+                 * If we observe the new cpu in task_rq_lock, the acquire will
+                 * pair with the WMB to ensure we must then also see migrating.
+                 */
                 if (likely(rq == task_rq(p) && !task_on_rq_migrating(p)))
                         return rq;
                 raw_spin_unlock(&rq->lock);