[PATCH] sched/deadline: Fix DL server divide-by-zero for inactive CPUs

Commit 4043f5498416 ("sched/deadline: Reject debugfs dl_server writes
for offline CPUs") rejects per-CPU DL server parameter updates once the
target CPU is offline. However, during CPU hot-unplug, the CPU is cleared
from cpu_active_mask before it is marked offline.

This leaves a window where cpu_online() is still true while
cpu_active() is already false. A debugfs write during this window passes
the cpu_online() check in sched_server_write_common() and reaches
dl_server_apply_params() with init=false.

dl_bw_cpus() counts the active CPUs in the root domain. For an isolated
CPU whose root-domain span contains only that CPU, it returns zero once
the CPU becomes inactive. If the server bandwidth is attached,
dl_server_apply_params() then passes this zero CPU count to __dl_sub()
and __dl_add(), both of which divide by the CPU count.

Using CPU1 with isolcpus=domain,1 and a temporary local hotplug pause
hook to stop the teardown after cpu_active_mask was cleared but before
the CPU became offline reproduced the state as:

dl_bw_cpus=0 attached=1 dl_b->bw=-1 total_bw=52428 span=1 active=0

Writing a new fair-server runtime while CPU1 was held in that state
triggered:

# echo 40000000 > /sys/kernel/debug/sched/fair_server/cpu1/runtime

Oops: divide error: 0000 [#1] SMP NOPTI
RIP: 0010:dl_server_apply_params+0x39d/0x400
Call Trace:
sched_server_write_common.isra.0+0x1d2/0x2d0
full_proxy_write+0x64/0x90
vfs_write+0xf7/0x540
ksys_write+0x6e/0xf0

Reject DL server parameter writes when the target CPU is inactive, not
only when it is offline.

Also update root-domain bandwidth in dl_server_apply_params() only while
the target CPU is active. This second check is necessary because CPU
hot-unplug can race with the debugfs path after its CPU state check and
before dl_server_apply_params() updates the bandwidth.

Keep the runqueue-local utilization update independent of cpu_active()
so that the local bandwidth state remains consistent if the CPU becomes
inactive during the parameter update.

With the fix, a write during the same hot-unplug window is rejected with
-EBUSY instead of reaching __dl_sub() or __dl_add() with a zero CPU
count.

Fixes: d741f297bcea ("sched/fair: Fair server interface")
Reported-by: Sashiko <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Cc: [email protected]
Signed-off-by: Hui Su <[email protected]>
---
 kernel/sched/deadline.c | 6 ++++--
 kernel/sched/debug.c    | 2 +-
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 200300043fa5..01adaba7ee3f 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1928,8 +1928,10 @@ int dl_server_apply_params(struct sched_dl_entity *dl_se, u64 runtime, u64 perio
 		__dl_add(dl_b, new_bw, cpus);
 		dl_se->dl_bw_attached = 1;
 	} else if (dl_se->dl_bw_attached) {
-		__dl_sub(dl_b, dl_se->dl_bw, cpus);
-		__dl_add(dl_b, new_bw, cpus);
+		if (cpu_active(cpu)) {
+			__dl_sub(dl_b, dl_se->dl_bw, cpus);
+			__dl_add(dl_b, new_bw, cpus);
+		}
 
 		dl_rq_change_utilization(rq, dl_se, new_bw);
 	}
diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 40584b27ea0c..ba60ff48dc3a 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -416,7 +416,7 @@ 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);
-- 
2.54.0