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Subject[PATCH 2/6] move_task_off_dead_cpu: take rq->lock around select_fallback_rq()
move_task_off_dead_cpu()->select_fallback_rq() reads/updates ->cpus_allowed
lockless. We can race with set_cpus_allowed() running in parallel.

Change it to take rq->lock around select_fallback_rq(). Note that it is not
trivial to move this spin_lock() into select_fallback_rq(), we must recheck
the task was not migrated after we take the lock and other callers do not
need this lock.

To avoid the races with other callers of select_fallback_rq() which rely on
TASK_WAKING, we also check p->state != TASK_WAKING and do nothing otherwise.
The owner of TASK_WAKING must update ->cpus_allowed and choose the correct
CPU anyway, and the subsequent __migrate_task() is just meaningless because
p->se.on_rq must be false.

Alternatively, we could change select_task_rq() to take rq->lock right
after it calls sched_class->select_task_rq(), but this looks a bit ugly.

Also, change it to not assume irqs are disabled and absorb __migrate_task_irq().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---

kernel/sched.c | 30 +++++++++++++++---------------
1 file changed, 15 insertions(+), 15 deletions(-)

--- 34-rc1/kernel/sched.c~2_MTODC_TAKE_RQ_LOCK 2010-03-15 09:40:16.000000000 +0100
+++ 34-rc1/kernel/sched.c 2010-03-15 09:40:31.000000000 +0100
@@ -5509,29 +5509,29 @@ static int migration_thread(void *data)
}

#ifdef CONFIG_HOTPLUG_CPU
-
-static int __migrate_task_irq(struct task_struct *p, int src_cpu, int dest_cpu)
-{
- int ret;
-
- local_irq_disable();
- ret = __migrate_task(p, src_cpu, dest_cpu);
- local_irq_enable();
- return ret;
-}
-
/*
* Figure out where task on dead CPU should go, use force if necessary.
*/
static void move_task_off_dead_cpu(int dead_cpu, struct task_struct *p)
{
- int dest_cpu;
-
+ struct rq *rq = cpu_rq(dead_cpu);
+ int needs_cpu, dest_cpu;
+ unsigned long flags;
again:
- dest_cpu = select_fallback_rq(dead_cpu, p);
+ local_irq_save(flags);
+
+ raw_spin_lock(&rq->lock);
+ needs_cpu = (task_cpu(p) == dead_cpu) && (p->state != TASK_WAKING);
+ if (needs_cpu)
+ dest_cpu = select_fallback_rq(dead_cpu, p);
+ raw_spin_unlock(&rq->lock);

/* It can have affinity changed while we were choosing. */
- if (unlikely(!__migrate_task_irq(p, dead_cpu, dest_cpu)))
+ if (needs_cpu)
+ needs_cpu = !__migrate_task(p, dead_cpu, dest_cpu);
+ local_irq_restore(flags);
+
+ if (unlikely(needs_cpu))
goto again;
}



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