[PATCH v4 04/26] sched: Let a task boosted by a deadline waiter change its policy

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

__sched_setscheduler() admits bandwidth for a change of or into
SCHED_DEADLINE: it calls sched_dl_overflow() where the new policy is
SCHED_DEADLINE or dl_task(p). dl_task() tests the priority, not the
policy: it holds for a task of another class boosted by a deadline waiter
blocked on an rt_mutex it holds. sched_dl_overflow() has no branch for a
change of such a task to another policy, which does not have the policy
to leave, and admits nothing: it returns -1, and the change fails with
EBUSY. sched_setscheduler() to SCHED_OTHER or SCHED_FIFO on the owner of
a priority-inheriting mutex fails while a deadline task waits on it, and
sysrq-n leaves the owner as it is.

Test the policy: a task of the class has it, and a boosted one has
nothing to admit or to leave. The deadline_bw_test selftest added later
in this series has the case, boosted_owner_policy.

Fixes: 332ac17ef5bf ("sched/deadline: Add bandwidth management for SCHED_DEADLINE tasks")
Cc: stable@vger.kernel.org
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---
 kernel/sched/syscalls.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index b8c036d03d6f..bdffe5452dc8 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -689,7 +689,9 @@ int __sched_setscheduler(struct task_struct *p,
 	 * If setscheduling to SCHED_DEADLINE (or changing the parameters
 	 * 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.
+	 * bandwidth it reserves. A task boosted into the class by priority
+	 * inheritance has nothing to admit nor to leave: the policy
+	 * decides, not the priority.
 	 *
 	 * Nothing to admit for parameters that are kept
 	 * (SCHED_FLAG_KEEP_PARAMS): they are the ones the task has, and
@@ -697,7 +699,8 @@ int __sched_setscheduler(struct task_struct *p,
 	 * change of its own that landed in between would leave them stale.
 	 */
 	if (!(attr->sched_flags & SCHED_FLAG_KEEP_PARAMS) &&
-	    (dl_policy(policy) || dl_task(p)) && sched_dl_overflow(p, policy, attr)) {
+	    (dl_policy(policy) || task_has_dl_policy(p)) &&
+	    sched_dl_overflow(p, policy, attr)) {
 		retval = -EBUSY;
 		goto unlock;
 	}
-- 
2.47.3