[PATCH v5 06/29] sched: Take cpuset_mutex for a deadline policy change only with pi

[Mathieu Desnoyers]

__sched_setscheduler() takes cpuset_mutex for a change that involves
SCHED_DEADLINE. On success, it releases it only when pi is set.
normalize_rt_tasks(), for sysrq-n, is the one caller with pi unset. It
runs under read_lock(&tasklist_lock). So sysrq-n takes the mutex in
atomic context for a deadline task, and never releases it. Every later
change that involves SCHED_DEADLINE then blocks forever.

Commit 710da3c8ea7d ("sched/core: Prevent race condition between cpuset
and __sched_setscheduler()") took the cpuset lock only when pi is set.
normalize_rt_tasks() cannot take it under tasklist_lock. Its changelog
accepts this: after sysrq-n, the deadline guarantees are gone anyway.
Commit 111cd11bbc54 ("sched/cpuset: Bring back cpuset_mutex") then took
the mutex only for deadline tasks. It replaced the pi test by a policy
test, and left the release under pi.

Fix this by taking the mutex for a deadline change only when pi is
set.

The policy test reads p->policy before the runqueue lock. It is checked
again under the lock only when the caller gave no policy. Otherwise,
the task can enter SCHED_DEADLINE in between. The call then takes it
out of the class without the mutex. A rebuild of the root domain
accounting could run at the same time. So check again under the
runqueue lock, and retry with the mutex.

sysrq-n on v7.4-rc1, with a deadline task and lockdep on:

  sysrq: Nice All RT Tasks
  BUG: sleeping function called from invalid context at kernel/locking/mutex.c:623
  in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 4356, name: deadline_switch
  locks held by deadline_switch/4356: 4, last CPU#3:
   ...
   #3: ffffffff83006098 (tasklist_lock){.+.+}-{3:3}, at: normalize_rt_tasks+0x34/0x150

A sched_setattr(SCHED_DEADLINE) after it never returned, in 2 VMs of 2.
The sysrq_normalize case of deadline_switch_test, added later in this
series, tests this with DEADLINE_TEST_SYSRQ=1. It passes with this
series.

Fixes: 111cd11bbc54 ("sched/cpuset: Bring back cpuset_mutex")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
---
 kernel/sched/syscalls.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index 56ce2352e89e..0bc778c12af4 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -561,9 +561,11 @@ int __sched_setscheduler(struct task_struct *p,
 
 	/*
 	 * SCHED_DEADLINE bandwidth accounting relies on stable cpusets
-	 * information.
+	 * information. Do not take cpuset_mutex when pi is unset: the
+	 * caller is normalize_rt_tasks(), which holds tasklist_lock and
+	 * cannot sleep.
 	 */
-	if (dl_policy(policy) || dl_policy(p->policy)) {
+	if (pi && (dl_policy(policy) || dl_policy(p->policy))) {
 		cpuset_locked = true;
 		cpuset_lock();
 	}
@@ -654,6 +656,15 @@ int __sched_setscheduler(struct task_struct *p,
 		goto recheck;
 	}
 
+	/*
+	 * The task entered SCHED_DEADLINE after the unlocked read of its
+	 * policy above, so cpuset_mutex was not taken. Retry with it.
+	 */
+	if (pi && !cpuset_locked && task_has_dl_policy(p)) {
+		task_rq_unlock(rq, p, &rf);
+		goto recheck;
+	}
+
 	oldprio = p->prio;
 
 	newprio = __normal_prio(policy, attr->sched_priority, attr->sched_nice);
-- 
2.47.3