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

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Message-ID: <20241005160723.GI33184@noisy.programming.kicks-ass.net>
Date: Sat, 5 Oct 2024 18:07:23 +0200
From: Peter Zijlstra <peterz@...radead.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>,
	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>,
	Frederic Weisbecker <frederic@...nel.org>,
	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

On Sat, Oct 05, 2024 at 06:04:44PM +0200, Peter Zijlstra wrote:
> On Fri, Oct 04, 2024 at 02:27:33PM -0400, Mathieu Desnoyers wrote:

> > +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.
> > +	 */

This should probably assert we're in a preemptible context. Otherwise
people will start using this in non-preemptible context and then we get
to unfuck things later.

> > +
> > +	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);
> 
> Is retirce_cb allowed to cmpxchg the thing?
> 
> > +		/* Busy-wait if node is found. */
> > +		while ((smp_load_acquire(&slot->addr)) == addr)	/* Load B */
> > +			cpu_relax();
> 
> This really should be using smp_cond_load_acquire()
> 
> > +	}
> > +}