[Devel] [RFC][PATCH 2/6] task's cpu information needs to be always correct

[Srivatsa Vaddagiri]

We rely very much on task_cpu(p) to be correct at all times, so that we
can correctly find the runqueue from which the task has to be removed or
added to.

There is however one place in the scheduler where this assumption of
task_cpu(p) being correct is broken. This patch fixes that piece of
code.

(Thanks to Balbir Singh for pointing this out to me)

Signed-off-by : Srivatsa Vaddagiri <vatsa at linux.vnet.ibm.com>

---
 kernel/sched.c |    8 +++++---
 1 files changed, 5 insertions(+), 3 deletions(-)

Index: current/kernel/sched.c
===================================================================
--- current.orig/kernel/sched.c	2007-06-09 15:07:17.000000000 +0530
+++ current/kernel/sched.c	2007-06-09 15:07:32.000000000 +0530
@@ -4624,7 +4624,7 @@
 static int __migrate_task(struct task_struct *p, int src_cpu, int dest_cpu)
 {
 	struct rq *rq_dest, *rq_src;
-	int ret = 0;
+	int ret = 0, on_rq;
 
 	if (unlikely(cpu_is_offline(dest_cpu)))
 		return ret;
@@ -4640,9 +4640,11 @@
 	if (!cpu_isset(dest_cpu, p->cpus_allowed))
 		goto out;
 
-	set_task_cpu(p, dest_cpu);
-	if (p->se.on_rq) {
+	on_rq = p->se.on_rq;
+	if (on_rq)
 		deactivate_task(rq_src, p, 0);
+	set_task_cpu(p, dest_cpu);
+	if (on_rq) {
 		activate_task(rq_dest, p, 0);
 		check_preempt_curr(rq_dest, p);
 	}
-- 
Regards,
vatsa
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