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    SubjectRe: [patch 2/2] sched: Protect sched_rr_get_param access to task->sched_class
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    On Wed, 2009-12-09 at 09:00 +0100, Peter Zijlstra wrote:
    > On Tue, 2009-12-08 at 20:24 +0000, Thomas Gleixner wrote:
    > > plain text document attachment
    > > (sched-fix-sched-get-rr-param-race.patch)
    > > sched_rr_get_param calls task->sched_class->get_rr_interval(task)
    > > without protection against a concurrent sched_setscheduler() call
    > > which modifies task->sched_class.
    > >
    > > Serialize the access with task_rq_lock(task);
    > >
    > > Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    > > ---
    > > kernel/sched.c | 4 ++++
    > > 1 file changed, 4 insertions(+)
    > >
    > > Index: linux-2.6-tip/kernel/sched.c
    > > ===================================================================
    > > --- linux-2.6-tip.orig/kernel/sched.c
    > > +++ linux-2.6-tip/kernel/sched.c
    > > @@ -6887,6 +6887,8 @@ SYSCALL_DEFINE2(sched_rr_get_interval, p
    > > {
    > > struct task_struct *p;
    > > unsigned int time_slice;
    > > + unsigned long flags;
    > > + struct rq *rq;
    > > int retval;
    > > struct timespec t;
    > >
    > > @@ -6903,7 +6905,9 @@ SYSCALL_DEFINE2(sched_rr_get_interval, p
    > > if (retval)
    > > goto out_unlock;
    > >
    > > + rq = task_rq_lock(p, &flags);
    > > time_slice = p->sched_class->get_rr_interval(p);
    > > + task_rq_unlock(rq, &flags);
    > >
    > > read_unlock(&tasklist_lock);
    > > jiffies_to_timespec(time_slice, &t);
    > >
    >
    > Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

    Oh, that was too quick, get_rr_interval_fair() actually takes the
    rq->lock, so lets add this:

    unsigned int get_rr_interval_fair(struct task_struct *task)
    {
    struct sched_entity *se = &task->se;
    - unsigned long flags;
    - struct rq *rq;
    unsigned int rr_interval = 0;

    /*
    * Time slice is 0 for SCHED_OTHER tasks that are on an otherwise
    * idle runqueue:
    */
    - rq = task_rq_lock(task, &flags);
    if (rq->cfs.load.weight)
    rr_interval = NS_TO_JIFFIES(sched_slice(&rq->cfs, se));
    - task_rq_unlock(rq, &flags);
    -
    return rr_interval;
    }




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