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Subjectsched: only update rq->clock while holding rq->lock
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Vatsa,

you said someone would send a patch to this effect, I don't want to
steal credit, but I think this patch is better posted sooner rather than
later.

---
Subject: sched: only update rq->clock while holding rq->lock
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
Date: Wed Oct 15 20:30:26 CEST 2008

Vatsa noticed rq->clock going funny and tracked it down to an update_rq_clock()
outside a rq->lock section.

This is a problem because things like double_rq_lock() update the rq->clock
value for both rqs. Therefore disabling interrupts isn't strong enough.

Reported-by: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
kernel/sched.c | 6 +-----
1 file changed, 1 insertion(+), 5 deletions(-)
Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -4448,12 +4448,8 @@ need_resched_nonpreemptible:
if (sched_feat(HRTICK))
hrtick_clear(rq);

- /*
- * Do the rq-clock update outside the rq lock:
- */
- local_irq_disable();
+ spin_lock_irq(&rq->lock);
update_rq_clock(rq);
- spin_lock(&rq->lock);
clear_tsk_need_resched(prev);

if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {



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