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    Subject[tip:sched/core] sched/core: Prevent race condition between cpuset and __sched_setscheduler()
    Commit-ID:  710da3c8ea7dfbd327920afd3831d8c82c42789d
    Gitweb: https://git.kernel.org/tip/710da3c8ea7dfbd327920afd3831d8c82c42789d
    Author: Juri Lelli <juri.lelli@redhat.com>
    AuthorDate: Fri, 19 Jul 2019 16:00:00 +0200
    Committer: Ingo Molnar <mingo@kernel.org>
    CommitDate: Thu, 25 Jul 2019 15:55:04 +0200

    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>
    ---
    include/linux/cpuset.h | 5 +++++
    kernel/cgroup/cpuset.c | 11 +++++++++++
    kernel/sched/core.c | 20 +++++++++++++++++---
    3 files changed, 33 insertions(+), 3 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 5c5014caa23c..c52bc91f882b 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 1af3d2dc6b29..1bceb22dac18 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -4698,6 +4698,9 @@ recheck:
    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 @@ change:
    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 @@ change:
    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 @@ change:

    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;
    }

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