linux-kernel - Re: [RFC PATCH v4 3/4] hazptr: Implement Hazard Pointers

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Message-ID: <6b27f3c3-47f7-402f-aa6c-b564e3db1d6a@efficios.com>
Date: Fri, 19 Dec 2025 17:19:55 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Joel Fernandes <joelagnelf@...dia.com>, Boqun Feng <boqun.feng@...il.com>
Cc: Joel Fernandes <joel@...lfernandes.org>,
 "Paul E. McKenney" <paulmck@...nel.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>,
 Will Deacon <will@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Alan Stern <stern@...land.harvard.edu>, John Stultz <jstultz@...gle.com>,
 Neeraj Upadhyay <Neeraj.Upadhyay@....com>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Frederic Weisbecker <frederic@...nel.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 v4 3/4] hazptr: Implement Hazard Pointers

On 2025-12-19 10:42, Joel Fernandes wrote:
> 
> IMHO the overflow case is "special" and should not happen often, otherwise
> things are "bad" anyway. I am not sure if this kind of complexity will be worth
> it unless we know HP forward-progress is a real problem. Also, since HP acquire
> will be short lived, are we that likely to not get past a temporary shortage of
> slots?

Given that we have context switch integration which moves the active
per-cpu slots to the overflow list, I can see that even not-so-special
users which get preempted or block regularly with active per-cpu slots
could theoretically end up preventing HP scan progress.

Providing HP scan progress guarantees is IMO an important aspect to
consider if we want to ensure the implementation does not cause subtle
HP scan stall when its amount of use will scale up.

> Perhaps the forward-progress problem should be rephrased to the following?: If a
> reader hit an overflow slot, it should probably be able to get a non-overflow
> slot soon, even if hazard pointer slots are over-subscribed.

Those are two distinct forward progress guarantees, and I think both are
important:

* Forward progress of HP readers,
* Forward progress of HP scan.

Thanks,

Mathieu

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