[PATCH v5 04/29] sched: Let a task boosted by a deadline waiter change its policy

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

A SCHED_OTHER or SCHED_FIFO task that holds an rt_mutex can be boosted
into the deadline class by a deadline waiter. Then a sched_setattr() or
sched_setscheduler() call on it fails with EBUSY, unless it asks for
SCHED_DEADLINE. sysrq-n would not reset it either.

The admission test runs "if setscheduling to SCHED_DEADLINE (or
changing the parameters of a SCHED_DEADLINE task)", as its comment
says. sched_dl_overflow() then handles three cases, by the task's
policy: the task enters, leaves or stays in SCHED_DEADLINE. But the
call site tests dl_task(p). That tests the priority, so a boosted task
passes it. For such a task and another policy, sched_dl_overflow()
matches no case and returns -1.

Fix this by testing the policy, with task_has_dl_policy(). A boosted
task has no bandwidth of its own to admit or release.

Now sysrq-n can take a boosted task out of the class.
normalize_rt_tasks() calls __sched_setscheduler() without pi, so the
task gets its normal priority. Nothing then drops the boost. At the
unlock, rt_mutex_setprio() sees no change of priority. The pi_se of the
task would keep pointing to the waiter, which can exit. So also drop the
boost in __setscheduler_dl_pi() when the task leaves the class, as
rt_mutex_setprio() does.

The boosted_owner_policy case of deadline_bw_test, added later in this
series, fails with EBUSY on v7.4-rc1. It passes with this series. The
sysrq_boosted_owner case of deadline_switch_test tests the sysrq-n path.

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 | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index 894f93a80d5d..56ce2352e89e 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -304,6 +304,13 @@ static inline void __setscheduler_dl_pi(int newprio, int policy,
 			scope->flags |= ENQUEUE_REPLENISH;
 		}
 	}
+
+	/*
+	 * Without pi, as from normalize_rt_tasks(), a boosted task can leave
+	 * the class. Drop its boost then, as rt_mutex_setprio() does.
+	 */
+	if (!dl_prio(newprio))
+		p->dl.pi_se = &p->dl;
 }
 #else /* !CONFIG_RT_MUTEXES */
 static inline void __setscheduler_dl_pi(int newprio, int policy,
@@ -691,12 +698,17 @@ int __sched_setscheduler(struct task_struct *p,
 	 * is available. Keep this check last. If a later check failed,
 	 * nothing would give back the bandwidth it reserves.
 	 *
+	 * Test the policy, not the priority. A task boosted into the class
+	 * by priority inheritance has no bandwidth of its own to admit or
+	 * release.
+	 *
 	 * 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 (!(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