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

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Message-ID: <34ec590c-b109-44a0-8bfe-8aafc6e7ad64@efficios.com>
Date: Fri, 27 Sep 2024 13:51:04 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
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 2024-09-27 19:23, Linus Torvalds wrote:
> 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

OK. I'll add the code/asm examples to the comment above ADDRESS_EQ().

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

If we use OPTIMIZER_HIDE_VAR() on both parameters, it indeed minimizes
the odds that someone get the order wrong, but it disallows using
ADDRESS_EQ() with a constant parameter
(e.g. ADDRESS_EQ(ptr, &mystruct)), which would be nice to cover. It
works fine with using OPTIMIZER_HIDE_VAR() on the first argument,
but it opens the door to misuses.

Perhaps there is a trick with compiler builtins we could do to only
use OPTIMIZER_HIDE_VAR() on non-constant arguments, but I can't get
it to work so far.

> 
>> 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".

Noted, thanks!

Mathieu

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