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

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Message-ID: <d86536d9-9c5a-48ab-abf3-3483e2e5e980@efficios.com>
Date: Fri, 27 Sep 2024 16:10:25 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>,
 Boqun Feng <boqun.feng@...il.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
 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 2024-09-27 21:23, Jonas Oberhauser wrote:
[...]
> That idea seems to be confirmed by this (atrocious, not to be copied!) 
> example:
> 
> int fct_escape_address_of_b(void)
> {
>      int *a, *b;
> 
>      do {
>          a = READ_ONCE(p);
>          asm volatile ("" : : : "memory");
>          b = READ_ONCE(p);
>      } while (a != b);
> 
>      // really really hide b
>      int **p = &b;
>      OPTIMIZER_HIDE_VAR(p);
> 
>      asm volatile ("" : : : "memory");
>      return *b;
> }
> 
> This also does not generate any additional instructions, unlike just 
> using OPTIMIZER_HIDE_VAR(b).
> 
> What is the advantage of defining OPTIMIZE_HIDE_VAR the way it currently 
> works instead of like above?

Did you try it on godbolt.org ? Does it have the intended effect ?
By the looks of it, you're just creating another version of @b called
"p", which is then never used and would be discarded by further
optimization. I'm unsure what you are trying to achieve here.

Thanks,

Mathieu

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