Re: [PATCH v10 2/5] sched: CGroup tagging interface for core scheduling

[Peter Zijlstra]

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.