[PATCH v8 7/8] sched/core: Prevent race condition between cpuset and __sched_setscheduler()

[Juri Lelli]

No synchronisation mechanism exists between the cpuset subsystem and
calls to function __sched_setscheduler(). As such, it is possible that
new root domains are created on the cpuset side while a deadline
acceptance test is carried out in __sched_setscheduler(), leading to a
potential oversell of CPU bandwidth.

Grab cpuset_rwsem read lock from core scheduler, so to prevent
situations such as the one described above from happening.

Signed-off-by: Juri Lelli <juri.le...@redhat.com>

---

v7->v8: use a percpu_rwsem read lock to avoid hotpath bottleneck issues
---
 include/linux/cpuset.h |  5 +++++
 kernel/cgroup/cpuset.c | 11 +++++++++++
 kernel/sched/core.c    | 33 ++++++++++++++++++++++++++-------
 3 files changed, 42 insertions(+), 7 deletions(-)

diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
index 7f1478c26a33..04c20de66afc 100644
--- a/include/linux/cpuset.h
+++ b/include/linux/cpuset.h
@@ -55,6 +55,8 @@ extern void cpuset_init_smp(void);
 extern void cpuset_force_rebuild(void);
 extern void cpuset_update_active_cpus(void);
 extern void cpuset_wait_for_hotplug(void);
+extern void cpuset_read_lock(void);
+extern void cpuset_read_unlock(void);
 extern void cpuset_cpus_allowed(struct task_struct *p, struct cpumask *mask);
 extern void cpuset_cpus_allowed_fallback(struct task_struct *p);
 extern nodemask_t cpuset_mems_allowed(struct task_struct *p);
@@ -176,6 +178,9 @@ static inline void cpuset_update_active_cpus(void)
 
 static inline void cpuset_wait_for_hotplug(void) { }
 
+static inline void cpuset_read_lock(void) { }
+static inline void cpuset_read_unlock(void) { }
+
 static inline void cpuset_cpus_allowed(struct task_struct *p,
                                       struct cpumask *mask)
 {
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index d92b351f89e3..f47b5c5c3c8b 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -334,6 +334,17 @@ static struct cpuset top_cpuset = {
  */
 
 DEFINE_STATIC_PERCPU_RWSEM(cpuset_rwsem);
+
+void cpuset_read_lock(void)
+{
+       percpu_down_read(&cpuset_rwsem);
+}
+
+void cpuset_read_unlock(void)
+{
+       percpu_up_read(&cpuset_rwsem);
+}
+
 static DEFINE_SPINLOCK(callback_lock);
 
 static struct workqueue_struct *cpuset_migrate_mm_wq;
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index acd6a9fe85bc..18e151c6b48d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4208,6 +4208,9 @@ static int __sched_setscheduler(struct task_struct *p,
                        return retval;
        }
 
+       if (pi)
+               cpuset_read_lock();
+
        /*
         * Make sure no PI-waiters arrive (or leave) while we are
         * changing the priority of the task:
@@ -4280,6 +4283,8 @@ static int __sched_setscheduler(struct task_struct *p,
        if (unlikely(oldpolicy != -1 && oldpolicy != p->policy)) {
                policy = oldpolicy = -1;
                task_rq_unlock(rq, p, &rf);
+               if (pi)
+                       cpuset_read_unlock();
                goto recheck;
        }
 
@@ -4338,8 +4343,10 @@ static int __sched_setscheduler(struct task_struct *p,
        preempt_disable();
        task_rq_unlock(rq, p, &rf);
 
-       if (pi)
+       if (pi) {
+               cpuset_read_unlock();
                rt_mutex_adjust_pi(p);
+       }
 
        /* Run balance callbacks after we've adjusted the PI chain: */
        balance_callback(rq);
@@ -4349,6 +4356,8 @@ static int __sched_setscheduler(struct task_struct *p,
 
 unlock:
        task_rq_unlock(rq, p, &rf);
+       if (pi)
+               cpuset_read_unlock();
        return retval;
 }
 
@@ -4433,10 +4442,15 @@ do_sched_setscheduler(pid_t pid, int policy, struct 
sched_param __user *param)
        rcu_read_lock();
        retval = -ESRCH;
        p = find_process_by_pid(pid);
-       if (p != NULL)
-               retval = sched_setscheduler(p, policy, &lparam);
+       if (!p) {
+               rcu_read_unlock();
+               goto exit;
+       }
+       get_task_struct(p);
        rcu_read_unlock();
-
+       retval = sched_setscheduler(p, policy, &lparam);
+       put_task_struct(p);
+exit:
        return retval;
 }
 
@@ -4564,10 +4578,15 @@ SYSCALL_DEFINE3(sched_setattr, pid_t, pid, struct 
sched_attr __user *, uattr,
        rcu_read_lock();
        retval = -ESRCH;
        p = find_process_by_pid(pid);
-       if (p != NULL)
-               retval = sched_setattr(p, &attr);
+       if (!p) {
+               rcu_read_unlock();
+               goto exit;
+       }
+       get_task_struct(p);
        rcu_read_unlock();
-
+       retval = sched_setattr(p, &attr);
+       put_task_struct(p);
+exit:
        return retval;
 }
 
-- 
2.17.2