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

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Message-ID: <ZwPmc_CXW15ilRaK@boqun-archlinux>
Date: Mon, 7 Oct 2024 06:47:31 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.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 3/4] hp: Implement Hazard Pointers

On Thu, Oct 03, 2024 at 09:30:53AM -0400, Mathieu Desnoyers wrote:
[...]
> > > +	/*
> > > +	 * Use RCU dereference without lockdep checks, because
> > > +	 * lockdep is not aware of HP guarantees.
> > > +	 */
> > > +	addr2 = rcu_access_pointer(*addr_p);	/* Load A */
> > 
> > Why rcu_access_pointer() instead of READ_ONCE()? Because you want to
> > mark the head of address dependency?
> 
> Yes, the intent here is to mark the address dependency and provide
> a publication guarantee similar to RCU pairing rcu_assign_pointer
> and rcu_dereference. Do you see any reason why READ_ONCE() would
> suffice here ?

READ_ONCE() also provides address dependencies. See the "DEPENDENCY
RELATIONS: data, addr, and ctrl" section in
tools/memory-model/Documentation/explanantion.txt.

Regards,
Boqun

> 
> Thanks,
> 
> Mathieu
>