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Subject[tip:sched/core] sched: Clarify ordering between task_rq_lock() and move_queued_task()
Commit-ID:  74b8a4cb6ce3685049ee124243a52238c5cabe55
Gitweb: http://git.kernel.org/tip/74b8a4cb6ce3685049ee124243a52238c5cabe55
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Tue, 17 Feb 2015 13:07:38 +0100
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 18 Feb 2015 14:27:28 +0100

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>
---
kernel/sched/core.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 1f37fe7..fd0a6ed 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);

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