linux-kernel - Re: [RFC PATCH v3 3/4] hazptr: Implement Hazard Pointers

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Message-ID: <a5cadedd-b462-4964-a1cf-f4623379b43e@efficios.com>
Date: Tue, 8 Oct 2024 13:05:29 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: 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>,
 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 v3 3/4] hazptr: Implement Hazard Pointers

On 2024-10-08 15:50, Mathieu Desnoyers wrote:
[...]

> +/* Retire the protected hazard pointer from @slot. */
> +static inline
> +void hazptr_retire(struct hazptr_slot *slot, void *addr)
> +{
> +	WARN_ON_ONCE(slot->addr != addr);
> +	smp_store_release(&slot->addr, NULL);
> +}

Actually, comparing this with the literature and past presentations
from Maged Michael, "retire" is not the appropriate name here.

With Hazard Pointers, AFAIU, the "retire" operation is similar to a
call_rcu() memory reclaim. It marks the object for eventual reclamation
when it is safe to do so.

The API here really just releases the recently protected hazard
pointer. So I will rename this to "hazptr_release()".

At this stage, this hazptr API does not implement any retire operation,
and only offers the hazptr_scan() for reader/updater synchronization,
leaving the actual reclaim to the caller.

Thanks,

Mathieu

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