From 74b8a4cb6ce3685049ee124243a52238c5cabe55 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Tue, 17 Feb 2015 13:07:38 +0100 Subject: [PATCH] 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 Signed-off-by: Peter Zijlstra (Intel) Cc: Linus Torvalds Cc: Oleg Nesterov Cc: Paul E. McKenney Link: http://lkml.kernel.org/r/20150217121258.GM5029@twins.programming.kicks-ass.net Signed-off-by: Ingo Molnar --- kernel/sched/core.c | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) 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);