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Date:   Thu, 4 Feb 2021 15:35:50 +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>,
        Tim Chen <tim.c.chen@linux.intel.com>,
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        Aubrey Li <aubrey.intel@gmail.com>, tglx@linutronix.de,
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        Valentin Schneider <valentin.schneider@arm.com>,
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        Junaid Shahid <junaids@google.com>, jsbarnes@google.com,
        chris.hyser@oracle.com, Ben Segall <bsegall@google.com>,
        Josh Don <joshdon@google.com>, Hao Luo <haoluo@google.com>,
        Tom Lendacky <thomas.lendacky@amd.com>
Subject: Re: [PATCH v10 2/5] sched: CGroup tagging interface for core
 scheduling
Message-ID: <YBwGRpnISmwkJ2i8@hirez.programming.kicks-ass.net>
References: <20210123011704.1901835-1-joel@joelfernandes.org>
 <20210123011704.1901835-3-joel@joelfernandes.org>
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In-Reply-To: <20210123011704.1901835-3-joel@joelfernandes.org>
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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.