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

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Message-ID: <CAHk-=wifOW0VEh6uL3sHSaAUA46YmPDS9Wh5HnNC2JyOiXVA=Q@mail.gmail.com>
Date: Thu, 26 Sep 2024 09:54:33 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Boqun Feng <boqun.feng@...il.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 <urezki@...il.com>, Steven 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>
Subject: Re: [RFC PATCH 1/4] hazptr: Add initial implementation of hazard pointers

On Thu, 26 Sept 2024 at 09:40, Jonas Oberhauser
<jonas.oberhauser@...weicloud.com> wrote:
>
> Boqun seems to be unhappy with a barrier though, because it would
> theoretically also forbid unrelated optimizations.

Well, doing a "barrier()" is kind of a big hammer thing, but honestly,
I don't think we've ever seen any real situation where it makes a
noticeable difference. Yes, it can pessimize compiler output more than
strictly necessary, but the kind of code generation issues it causes
tends to be the non-problematic kind (and particularly the kind that
even a trivial OoO core will deal with well).

We do have some more directed compiler barriers available, and this
code might be able to use OPTIMIZER_HIDE_VAR() for example. It's kind
of a "single variable value barrier".

Honestly, we don't use it much. It just tends to be _too_specific. But
it is there if somebody wants to use it.

> But I have not seen any evidence that there are any unrelated
> optimizations going on in the first place that would be forbidden by this.

Compared to something like "smp_mb()", which is not just a compiler
barrier but also generates typically very expensive instructions that
completely mess with an OoO core, a regular compiler barrier is a
complete non-issue. When you have those two close to each other, you'd
have to make up some very odd situation where the plain "barrier()" is
even noticeable.

               Linus