linux-kernel - Re: [RFC PATCH 1/4] hazptr: Add initial implementation of hazard pointers

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Message-ID: <CAHk-=wgBgh5U+dyNaN=+XCdcm2OmgSRbcH4Vbtk8i5ZDGwStSA@mail.gmail.com>
Date: Fri, 27 Sep 2024 10:23:24 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Boqun Feng <boqun.feng@...il.com>, 
	Jonas Oberhauser <jonas.oberhauser@...weicloud.com>, linux-kernel@...r.kernel.org, 
	rcu@...r.kernel.org, linux-mm@...ck.org, lkmm@...ts.linux.dev, 
	"Paul E. McKenney" <paulmck@...nel.org>, Frederic Weisbecker <frederic@...nel.org>, 
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>, Joel Fernandes <joel@...lfernandes.org>, 
	Josh Triplett <josh@...htriplett.org>, "Uladzislau Rezki (Sony)" <urezki@...il.com>, rostedt <rostedt@...dmis.org>, 
	Lai Jiangshan <jiangshanlai@...il.com>, Zqiang <qiang.zhang1211@...il.com>, 
	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>, 
	Waiman Long <longman@...hat.com>, Mark Rutland <mark.rutland@....com>, 
	Thomas Gleixner <tglx@...utronix.de>, Kent Overstreet <kent.overstreet@...il.com>, 
	Vlastimil Babka <vbabka@...e.cz>, maged.michael@...il.com, 
	Neeraj Upadhyay <neeraj.upadhyay@....com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [RFC PATCH 1/4] hazptr: Add initial implementation of hazard pointers

On Fri, 27 Sept 2024 at 10:17, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
>
> The barrier() is ineffective at fixing the issue.
> It does not prevent the compiler CSE from losing the
> address dependency:

Ok. Thanks for actually specifying code.

That needs to be

 (a) in a comment

 (b) the value barrier needs to be on *both* values so that the order
of the equality testing doesn't matter.

> I'm preparing a small series that aims to show how a minimal
> hazard pointer implementation can help improve common scenarios:

I want actual numbers on real loads. Just so you know.  Not "this can
help". But "this actually really _does_ help".

                    Linus