linux-kernel - Re: [RFC PATCH v2 3/4] hp: Implement Hazard Pointers

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Message-ID: <ZwEgtU8MFPfGmEfA@pavilion.home>
Date: Sat, 5 Oct 2024 13:19:17 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Nicholas Piggin <npiggin@...il.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Will Deacon <will@...nel.org>,
	Alan Stern <stern@...land.harvard.edu>,
	John Stultz <jstultz@...gle.com>,
	Neeraj Upadhyay <Neeraj.Upadhyay@....com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Josh Triplett <josh@...htriplett.org>,
	Uladzislau Rezki <urezki@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>, Ingo Molnar <mingo@...hat.com>,
	Waiman Long <longman@...hat.com>,
	Mark Rutland <mark.rutland@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vlastimil Babka <vbabka@...e.cz>, maged.michael@...il.com,
	Mateusz Guzik <mjguzik@...il.com>,
	Jonas Oberhauser <jonas.oberhauser@...weicloud.com>,
	rcu@...r.kernel.org, linux-mm@...ck.org, lkmm@...ts.linux.dev
Subject: Re: [RFC PATCH v2 3/4] hp: Implement Hazard Pointers

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
>