[Devel] [RFC] [PATCH 1/3] task_cpu(p) needs to be correct always

[Srivatsa Vaddagiri]

We rely very much on task_cpu(p) to be correct at all times, so that we can
correctly find the task_grp_rq 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 in.ibm.com>

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

Index: linux-2.6.21-rc7/kernel/sched.c
===================================================================
--- linux-2.6.21-rc7.orig/kernel/sched.c	2007-05-23 20:46:41.000000000 +0530
+++ linux-2.6.21-rc7/kernel/sched.c	2007-05-23 20:48:26.000000000 +0530
@@ -4571,7 +4571,7 @@
 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;
@@ -4587,9 +4587,11 @@
 	if (!cpu_isset(dest_cpu, p->cpus_allowed))
 		goto out;
 
-	set_task_cpu(p, dest_cpu);
-	if (p->on_rq) {
+	on_rq = p->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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