[PATCH v3 03/24] sched/deadline: Do not admit bandwidth for parameters that are kept

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

sys_sched_setattr() fills a SCHED_FLAG_KEEP_PARAMS request in from the
task itself (get_params()), with no lock held, and
__sched_setscheduler() then skips __setscheduler_params(): the task
keeps the parameters it has, and its dl_bw with them.
sched_dl_overflow() runs all the same.

A parameter change of the task's own that lands in between leaves the
request stale, and dl_param_changed() takes it for a change: total_bw of
the root domain, and this_bw and running_bw of its runqueue through
dl_change_utilization(), then hold a bandwidth the task does not have,
and the release at its 0-lag time, which takes the task's own dl_bw,
leaves the difference behind.

There is nothing to admit for parameters that are kept: skip the check.

Found by reading, after the Sashiko AI review of the first posting of
this series raised the case the previous patch refuses. Not reproduced:
it needs a parameter change of a deadline task to land inside the window
of a SCHED_FLAG_KEEP_PARAMS request on the same task.

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 | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index 6819f0fea3c5..b8c036d03d6f 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -690,8 +690,14 @@ int __sched_setscheduler(struct task_struct *p,
 	 * of a SCHED_DEADLINE task) we need to check if enough bandwidth
 	 * is available. Last of the checks: nothing gives back the
 	 * bandwidth it reserves.
+	 *
+	 * Nothing to admit for parameters that are kept
+	 * (SCHED_FLAG_KEEP_PARAMS): they are the ones the task has, and
+	 * sys_sched_setattr() read them from it with no lock held, so a
+	 * change of its own that landed in between would leave them stale.
 	 */
-	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