[PATCH 1/4] sched: consult online mask instead of active in select_fallback_rq()

[Tejun Heo]

If called after sched_class chooses a CPU which isn't in a task's
cpus_allowed mask, select_fallback_rq() can end up migrating a task
which is bound to an !active but online cpu to an active cpu.  This is
dangerous because active is cleared before CPU_DOWN_PREPARE is called
and subsystems expect affinities of kthreads and other tasks to be
maintained till their CPU_DOWN_PREPARE callbacks are complete.

Consult cpu_online_mask instead.

This problem is triggered by cmwq.  During CPU_DOWN_PREPARE, hotplug
callback creates the trustee kthread and kthread_bind()s it to the
target cpu, and the trustee is expected to run on that cpu.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
---
 kernel/sched.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index b531d79..aca4a20 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2262,12 +2262,12 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
 	const struct cpumask *nodemask = cpumask_of_node(cpu_to_node(cpu));
 
 	/* Look for allowed, online CPU in same node. */
-	for_each_cpu_and(dest_cpu, nodemask, cpu_active_mask)
+	for_each_cpu_and(dest_cpu, nodemask, cpu_online_mask)
 		if (cpumask_test_cpu(dest_cpu, &p->cpus_allowed))
 			return dest_cpu;
 
 	/* Any allowed, online CPU? */
-	dest_cpu = cpumask_any_and(&p->cpus_allowed, cpu_active_mask);
+	dest_cpu = cpumask_any_and(&p->cpus_allowed, cpu_online_mask);
 	if (dest_cpu < nr_cpu_ids)
 		return dest_cpu;
 
-- 
1.6.4.2

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