[PATCH v3 1/8] sched/deadline: Reject debugfs dl_server writes for not-active CPUs.

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

sched_server_write_common() checks if the CPU is online before allowing
fair_server updates. This is important because updating the fair_server
runtime/ period will use dl_bw_cpus() will return only the number of
CPUs which are also marked active.

The CPU is marked active as the last step during CPU-hotplug. If the CPU
has been marked "online" but not yet "active" or is stuck between those
two states then any update of period/ runtime will lead to a divide-by-0
error.

Let sched_server_write_common() check of active CPUs instead of online.
Additonaly, let dl_server_apply_params() return an error if dl_bw_cpus()
return 0 CPUs to avoid the divide error.

Fixes: 4043f54984161 ("sched/deadline: Reject debugfs dl_server writes for offline CPUs")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/sched/deadline.c | 3 +++
 kernel/sched/debug.c    | 2 +-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index dfae08625342e..10beb54281881 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1920,6 +1920,9 @@ int dl_server_apply_params(struct sched_dl_entity *dl_se, u64 runtime, u64 perio
 	cpus = dl_bw_cpus(cpu);
 	cap = dl_bw_capacity(cpu);
 
+	if (!cpus)
+		return -EBUSY;
+
 	if (__dl_overflow(dl_b, cap, old_bw, new_bw))
 		return -EBUSY;
 
diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index e6a3b516c703b..1545139e52443 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -410,7 +410,7 @@ static ssize_t sched_server_write_common(struct file *filp, const char __user *u
 			return  -EINVAL;
 		}
 
-		if (!cpu_online(cpu_of(rq)))
+		if (!cpu_active(cpu_of(rq)))
 			return -EBUSY;
 
 		update_rq_clock(rq);

-- 
2.55.0