[PATCH v5 03/29] sched/deadline: Do not admit bandwidth for parameters that are kept

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

__sched_setscheduler() runs the deadline admission test on a request
with SCHED_FLAG_KEEP_PARAMS. But the task keeps its parameters: the
call skips __setscheduler_params().

Commit f4dddf90d58d ("sched: Skip priority checks with
SCHED_FLAG_KEEP_PARAMS") made sys_sched_setattr() copy the task's own
parameters into such a request. Then the priority checks pass.
get_params() copies them without a lock.

Another call can change the task's parameters before
__sched_setscheduler() takes the runqueue lock. The request is then
stale, and dl_param_changed() takes it for a change. sched_dl_overflow()
would put the stale bandwidth in place of the task's own in total_bw,
and in this_bw for a sleeping task. But the task keeps its own dl_bw.
Its release at the 0-lag time takes that dl_bw, and the difference
stays behind.

Fix this by skipping sched_dl_overflow() for SCHED_FLAG_KEEP_PARAMS.
There is nothing to admit for parameters that are kept.

Found by reading, after the Sashiko AI review of the first posting of
this series raised the boosted case of the previous patch. Not
reproduced: the parameter change must land inside that window.

Fixes: a509a7cd7974 ("sched/uclamp: Extend sched_setattr() to support utilization clamping")
Cc: stable@vger.kernel.org
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---
 kernel/sched/syscalls.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index 0e8b5c7324f6..894f93a80d5d 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -690,8 +690,13 @@ int __sched_setscheduler(struct task_struct *p,
 	 * of a SCHED_DEADLINE task) we need to check if enough bandwidth
 	 * is available. Keep this check last. If a later check failed,
 	 * nothing would give back the bandwidth it reserves.
+	 *
+	 * With SCHED_FLAG_KEEP_PARAMS, the task keeps its parameters, so
+	 * there is nothing to admit. The parameters in attr may also be
+	 * stale: sys_sched_setattr() read them from the task without a lock.
 	 */
-	if ((dl_policy(policy) || dl_task(p)) && sched_dl_overflow(p, policy, attr)) {
+	if (!(attr->sched_flags & SCHED_FLAG_KEEP_PARAMS) &&
+	    (dl_policy(policy) || dl_task(p)) && sched_dl_overflow(p, policy, attr)) {
 		retval = -EBUSY;
 		goto unlock;
 	}
-- 
2.47.3