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Date:   Wed, 25 Nov 2020 12:10:14 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Joel Fernandes (Google)" <joel@joelfernandes.org>
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Subject: Re: [PATCH -tip 22/32] sched: Split the cookie and setup per-task
 cookie on fork
Message-ID: <20201125111014.GS2414@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:
> +void sched_core_tag_requeue(struct task_struct *p, unsigned long cookie, bool group)
> +{
> +	if (!p)
> +		return;
> +
> +	if (group)
> +		p->core_group_cookie = cookie;
> +	else
> +		p->core_task_cookie = cookie;
> +
> +	/* Use up half of the cookie's bits for task cookie and remaining for group cookie. */
> +	p->core_cookie = (p->core_task_cookie <<
> +				(sizeof(unsigned long) * 4)) + p->core_group_cookie;

This seems dangerous; afaict there is nothing that prevents cookie
collision.