sched/core: Prevent race condition between cpuset and __sched_setscheduler()

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.

The only exception is normalize_rt_tasks() which needs to work under
tasklist_lock and can't therefore grab cpuset_rwsem. We are fine with
this, as this function is only called by sysrq and, if that gets
triggered, DEADLINE guarantees are already gone out of the window
anyway.

Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: bristot@redhat.com
Cc: claudio@evidence.eu.com
Cc: lizefan@huawei.com
Cc: longman@redhat.com
Cc: luca.abeni@santannapisa.it
Cc: mathieu.poirier@linaro.org
Cc: rostedt@goodmis.org
Cc: tj@kernel.org
Cc: tommaso.cucinotta@santannapisa.it
Link: https://lkml.kernel.org/r/20190719140000.31694-9-juri.lelli@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
index 7f1478c26a33a9cabae6f12bcd320db0ec9e5c81..04c20de66afc2136e5bf3251b4b1780e30a1b4be 100644 (file)
--- 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 5c5014caa23c3998640a374394893433f6adcece..c52bc91f882b29e44ea8eb56fd0ba9889c6a86df 100644 (file)
--- 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 1af3d2dc6b294ae0437661d2663b7bcfa265895a..1bceb22dac1807c91c9e7c2142d4ea0c798b9cca 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4698,6 +4698,9 @@ 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:
@@ -4772,6 +4775,8 @@ 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;
        }
 
@@ -4832,8 +4837,10 @@ 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);
@@ -4843,6 +4850,8 @@ static int __sched_setscheduler(struct task_struct *p,
 
 unlock:
        task_rq_unlock(rq, p, &rf);
+       if (pi)
+               cpuset_read_unlock();
        return retval;
 }
 
@@ -4927,10 +4936,15 @@ 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 (likely(p))
+               get_task_struct(p);
        rcu_read_unlock();
 
+       if (likely(p)) {
+               retval = sched_setscheduler(p, policy, &lparam);
+               put_task_struct(p);
+       }
+
        return retval;
 }