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SubjectRe: [PATCH v6 2/2] sched/uclamp: Protect uclamp fast path code with static key
On Tue, Jun 30, 2020 at 12:21:23PM +0100, Qais Yousef wrote:
> @@ -993,10 +1013,38 @@ static inline void uclamp_rq_dec_id(struct rq *rq, struct task_struct *p,
>
> lockdep_assert_held(&rq->lock);
>
> + /*
> + * If sched_uclamp_used was enabled after task @p was enqueued,
> + * we could end up with unbalanced call to uclamp_rq_dec_id().
> + *
> + * In this case the uc_se->active flag should be false since no uclamp
> + * accounting was performed at enqueue time and we can just return
> + * here.
> + *
> + * Need to be careful of the following enqeueue/dequeue ordering
> + * problem too
> + *
> + * enqueue(taskA)
> + * // sched_uclamp_used gets enabled
> + * enqueue(taskB)
> + * dequeue(taskA)
> + * // Must not decrement bukcet->tasks here
> + * dequeue(taskB)
> + *
> + * where we could end up with stale data in uc_se and
> + * bucket[uc_se->bucket_id].
> + *
> + * The following check here eliminates the possibility of such race.
> + */
> + if (unlikely(!uc_se->active))
> + return;
> +
> bucket = &uc_rq->bucket[uc_se->bucket_id];
> +
> SCHED_WARN_ON(!bucket->tasks);
> if (likely(bucket->tasks))
> bucket->tasks--;
> +
> uc_se->active = false;
>
> /*

> @@ -1221,6 +1289,8 @@ static void __setscheduler_uclamp(struct task_struct *p,
> if (likely(!(attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)))
> return;
>
> + static_branch_enable(&sched_uclamp_used);
> +
> if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN) {
> uclamp_se_set(&p->uclamp_req[UCLAMP_MIN],
> attr->sched_util_min, true);
> @@ -7387,6 +7457,8 @@ static ssize_t cpu_uclamp_write(struct kernfs_open_file *of, char *buf,
> if (req.ret)
> return req.ret;
>
> + static_branch_enable(&sched_uclamp_used);
> +
> mutex_lock(&uclamp_mutex);
> rcu_read_lock();
>

There's a fun race described in 9107c89e269d ("perf: Fix race between
event install and jump_labels"), are we sure this isn't also susceptible
to something similar?

I suspect not, but I just wanted to make sure.

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