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SubjectRe: [PATCH v10 2/5] sched: CGroup tagging interface for core scheduling
On Fri, Jan 22, 2021 at 08:17:01PM -0500, Joel Fernandes (Google) wrote:
> +static inline void __sched_core_erase_cookie(struct sched_core_cookie *cookie)
> +{
> + lockdep_assert_held(&sched_core_cookies_lock);
> +
> + /* Already removed */
> + if (RB_EMPTY_NODE(&cookie->node))
> + return;
> +
> + rb_erase(&cookie->node, &sched_core_cookies);
> + RB_CLEAR_NODE(&cookie->node);
> +}
> +
> +/* Called when a task no longer points to the cookie in question */
> +static void sched_core_put_cookie(struct sched_core_cookie *cookie)
> +{
> + unsigned long flags;
> +
> + if (!cookie)
> + return;
> +
> + if (refcount_dec_and_test(&cookie->refcnt)) {
> + raw_spin_lock_irqsave(&sched_core_cookies_lock, flags);
> + __sched_core_erase_cookie(cookie);
> + raw_spin_unlock_irqrestore(&sched_core_cookies_lock, flags);
> + kfree(cookie);
> + }
> +}

> +static void __sched_core_update_cookie(struct task_struct *p)
> +{

> + raw_spin_lock(&sched_core_cookies_lock);

> + /*
> + * Cookie exists, increment refcnt. If refcnt is currently 0,
> + * we're racing with a put() (refcnt decremented but cookie not
> + * yet removed from the tree). In this case, we can simply
> + * perform the removal ourselves and retry.
> + * sched_core_put_cookie() will still function correctly.
> + */
> + if (unlikely(!refcount_inc_not_zero(&match->refcnt))) {
> + __sched_core_erase_cookie(match);
> + goto retry;
> + }

refcount_dec_and_lock() avoids that complication.

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