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

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Message-ID: <2b2aea37-06fe-40cb-8458-9408406ebda6@efficios.com>
Date: Thu, 26 Sep 2024 08:16:08 +0200
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: 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 <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>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 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 2024-09-25 15:20, Mathieu Desnoyers wrote:
[...]
> static inline
> bool same_ptr(void *a, void *b)
> {
>      asm goto (
>          "cmpq %[a], %[b]\n\t"
>          "jne %l[ne]\n\t"
>          : : [a] "r" (a), [b] "r" (b)
>          : : ne);
>      return true;
> ne:
>      return false;
> }

Based on the information provided in this email thread,
it appears the only concern when it comes to comparing a
pointer loaded by rcu_dereference() with another pointer
is the possibility of compiler optimizations.

In the specific case of hazard pointer hpref_hp_get(), this
function does both loads which are then compared with one
another. Therefore, it is not possible for the user to
provide a comparison value known at compile-time, except
in the unlikely scenario where the hazard pointer would
be const, which does not really make much sense.

Therefore, just using rcu_dereference() for the second
load should be fine.

Thanks,

Mathieu

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