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Date: Sat, 5 Oct 2024 13:19:17 +0200
From: Frederic Weisbecker <frederic@kernel.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Boqun Feng <boqun.feng@gmail.com>, linux-kernel@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Nicholas Piggin <npiggin@gmail.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	"Paul E. McKenney" <paulmck@kernel.org>,
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	Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>,
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	Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>,
	rcu@vger.kernel.org, linux-mm@kvack.org, lkmm@lists.linux.dev
Subject: Re: [RFC PATCH v2 3/4] hp: Implement Hazard Pointers
Message-ID: <ZwEgtU8MFPfGmEfA@pavilion.home>
References: <20241004182734.1761555-1-mathieu.desnoyers@efficios.com>
 <20241004182734.1761555-4-mathieu.desnoyers@efficios.com>
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In-Reply-To: <20241004182734.1761555-4-mathieu.desnoyers@efficios.com>

Le Fri, Oct 04, 2024 at 02:27:33PM -0400, Mathieu Desnoyers a écrit :
> +void hp_scan(struct hp_slot __percpu *percpu_slots, void *addr,
> +	     void (*retire_cb)(int cpu, struct hp_slot *slot, void *addr))
> +{
> +	int cpu;
> +
> +	/*
> +	 * Store A precedes hp_scan(): it unpublishes addr (sets it to
> +	 * NULL or to a different value), and thus hides it from hazard
> +	 * pointer readers.
> +	 */
> +
> +	if (!addr)
> +		return;
> +	/* Memory ordering: Store A before Load B. */
> +	smp_mb();
> +	/* Scan all CPUs slots. */
> +	for_each_possible_cpu(cpu) {
> +		struct hp_slot *slot = per_cpu_ptr(percpu_slots, cpu);
> +
> +		if (retire_cb && smp_load_acquire(&slot->addr) == addr)	/* Load B */
> +			retire_cb(cpu, slot, addr);
> +		/* Busy-wait if node is found. */
> +		while ((smp_load_acquire(&slot->addr)) == addr)	/* Load B */
> +			cpu_relax();

You agree that having a single possible per-cpu pointer per context and a busy
waiting update side pointer release can't be a general purpose hazard pointer
implementation, right? :-)

Thanks.

> +	}
> +}
> -- 
> 2.39.2
>