Commit 74b8a4cb authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar

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: default avatarKirill Tkhai <ktkhai@parallels.com>
Signed-off-by: default avatarPeter 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.netSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent e07e0d4c
...@@ -341,6 +341,22 @@ static struct rq *task_rq_lock(struct task_struct *p, unsigned long *flags) ...@@ -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); raw_spin_lock_irqsave(&p->pi_lock, *flags);
rq = task_rq(p); rq = task_rq(p);
raw_spin_lock(&rq->lock); 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))) if (likely(rq == task_rq(p) && !task_on_rq_migrating(p)))
return rq; return rq;
raw_spin_unlock(&rq->lock); raw_spin_unlock(&rq->lock);
......
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