netdev - Re: [PATCH 1/8] Introduce simple hazard pointers

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Message-ID: <aFwQTO-_t-P5rLUp@Mac.home>
Date: Wed, 25 Jun 2025 08:05:48 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: linux-kernel@...r.kernel.org, rcu@...r.kernel.org, lkmm@...ts.linux.dev,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>,
	Waiman Long <longman@...hat.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Josh Triplett <josh@...htriplett.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Uladzislau Rezki <urezki@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang@...ux.dev>, Breno Leitao <leitao@...ian.org>,
	aeh@...a.com, netdev@...r.kernel.org, edumazet@...gle.com,
	jhs@...atatu.com, kernel-team@...a.com,
	Erik Lundgren <elundgren@...a.com>
Subject: Re: [PATCH 1/8] Introduce simple hazard pointers

On Wed, Jun 25, 2025 at 10:25:23AM -0400, Mathieu Desnoyers wrote:
> On 2025-06-24 23:10, Boqun Feng wrote:
> [...]
> > +
> > +static inline void shazptr_clear(struct shazptr_guard guard)
> > +{
> > +	/* Only clear the slot when the outermost guard is released */
> > +	if (likely(!guard.use_wildcard))
> > +		smp_store_release(guard.slot, NULL); /* Pair with ACQUIRE at synchronize_shazptr() */
> 
> How is the wildcard ever cleared ?
> 

The outermost shazptr_guard will have .use_wildcard being false, so it
will clear the wildcard. E.g.

	g1 = shazptr_acqure(a); // g1->use_wildcard is false
				// this cpu's hazptr slot is 'a'.
	g2 = shazptr_acqure(b); // g2->use_wildcard is true
				// this cpu's hazptr slot becomes
				// WILDCARD.

	shazptr_clear(g2);	// do nothing
	shazptr_clear(g1);	// clear this cpu's hazptr slot to NULL.

Regards,
Boqun

> Thanks,
> 
> Mathieu
> 
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com