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Date:   Wed, 25 Nov 2020 14:03:22 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc:     Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
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        "Paul E. McKenney" <paulmck@kernel.org>,
        Tim Chen <tim.c.chen@intel.com>
Subject: Re: [PATCH -tip 22/32] sched: Split the cookie and setup per-task
 cookie on fork
Message-ID: <20201125130322.GW2414@hirez.programming.kicks-ass.net>
References: <20201117232003.3580179-1-joel@joelfernandes.org>
 <20201117232003.3580179-23-joel@joelfernandes.org>
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On Tue, Nov 17, 2020 at 06:19:52PM -0500, Joel Fernandes (Google) wrote:
> +static bool sched_core_get_task_cookie(unsigned long cookie)
> +{
> +	struct sched_core_cookie *ptr = (struct sched_core_cookie *)cookie;
> +
> +	/*
> +	 * NOTE: sched_core_put() is not done by put_task_cookie(). Instead, it
> +	 * is done after the stopper runs.
> +	 */
> +	sched_core_get();
> +	return refcount_inc_not_zero(&ptr->refcnt);

See below, but afaict this should be refcount_inc().

> +}

> +	/*
> +	 * 		t1		joining		t2
> +	 * CASE 1:
> +	 * before	0				0
> +	 * after	new cookie			new cookie
> +	 *
> +	 * CASE 2:
> +	 * before	X (non-zero)			0
> +	 * after	0				0
> +	 *
> +	 * CASE 3:
> +	 * before	0				X (non-zero)
> +	 * after	X				X
> +	 *
> +	 * CASE 4:
> +	 * before	Y (non-zero)			X (non-zero)
> +	 * after	X				X
> +	 */
> +	if (!t1->core_task_cookie && !t2->core_task_cookie) {
> +		/* CASE 1. */
> +		cookie = sched_core_alloc_task_cookie();
> +		if (!cookie)
> +			goto out_unlock;
> +
> +		/* Add another reference for the other task. */
> +		if (!sched_core_get_task_cookie(cookie)) {

afaict this should be refcount_inc(), as this can never fail and if it
does, it's an error.

> +			ret = -EINVAL;
> +			goto out_unlock;
> +		}
> +
> +		wr.tasks[0] = t1;
> +		wr.tasks[1] = t2;
> +		wr.cookies[0] = wr.cookies[1] = cookie;
> +
> +	} else if (t1->core_task_cookie && !t2->core_task_cookie) {
> +		/* CASE 2. */
> +		sched_core_put_task_cookie(t1->core_task_cookie);
> +		sched_core_put_after_stopper = true;
> +
> +		wr.tasks[0] = t1; /* Reset cookie for t1. */
> +
> +	} else if (!t1->core_task_cookie && t2->core_task_cookie) {
> +		/* CASE 3. */
> +		if (!sched_core_get_task_cookie(t2->core_task_cookie)) {

afaict this can never fail either, because you're calling in here with a
reference on t2

> +			ret = -EINVAL;
> +			goto out_unlock;
> +		}
> +
> +		wr.tasks[0] = t1;
> +		wr.cookies[0] = t2->core_task_cookie;
> +
> +	} else {
> +		/* CASE 4. */
> +		if (!sched_core_get_task_cookie(t2->core_task_cookie)) {

Same.

> +			ret = -EINVAL;
> +			goto out_unlock;
> +		}
> +		sched_core_put_task_cookie(t1->core_task_cookie);
> +		sched_core_put_after_stopper = true;
> +
> +		wr.tasks[0] = t1;
> +		wr.cookies[0] = t2->core_task_cookie;
> +	}