linux-kernel - Re: [PATCH 1/2] compiler.h: Introduce ptr

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Message-ID: <c1fbec54-a2e3-418b-b1fb-14fd16092613@efficios.com>
Date: Thu, 3 Oct 2024 14:00:40 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: David Laight <David.Laight@...LAB.COM>,
 'Alan Stern' <stern@...land.harvard.edu>
Cc: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 "Paul E. McKenney" <paulmck@...nel.org>, Will Deacon <will@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>, Boqun Feng <boqun.feng@...il.com>,
 John Stultz <jstultz@...gle.com>, Neeraj Upadhyay <Neeraj.Upadhyay@....com>,
 Frederic Weisbecker <frederic@...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>,
 Ingo Molnar <mingo@...hat.com>, Waiman Long <longman@...hat.com>,
 Mark Rutland <mark.rutland@....com>, Thomas Gleixner <tglx@...utronix.de>,
 Vlastimil Babka <vbabka@...e.cz>,
 "maged.michael@...il.com" <maged.michael@...il.com>,
 Mateusz Guzik <mjguzik@...il.com>, Gary Guo <gary@...yguo.net>,
 "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
 "linux-mm@...ck.org" <linux-mm@...ck.org>,
 "lkmm@...ts.linux.dev" <lkmm@...ts.linux.dev>
Subject: Re: [PATCH 1/2] compiler.h: Introduce ptr_eq() to preserve address
 dependency

On 2024-10-03 19:07, David Laight wrote:
> ...
>> What _does_ work however are the following two approaches:
>>
>> 1) Perform the equality check on the original variables, creating
>> new versions (with OPTIMIZER_HIDE_VAR) of both variables for the
>> rest of their use, therefore making sure the pointer dereference
>> are not derived from versions of the variables which were compared
>> with another pointer. (as suggested by Boqun)
> 
> If that is
> 	a1 = a; OPTIMISER_HIDE_VAR(a1);
> 	b1 = b; OPTIMISER_HIDE_BAR(b1);
> 	if (a != b}
> 		return;
> 	// code using a1 and b1
> then can't the compiler first flip it to:
> 	if (a != b)
> 		return;
> 	a1 = a; OPTIMISER_HIDE_VAR(a1);
> 	b1 = b; OPTIMISER_HIDE_VAR(b1);
> and then replace the last line with:
> 	b1 = a; OPTIMISER_HIDE_VAR(b1);
> which isn't intended at all.

Good point, so I suspect Boqun's ADDRESS_EQ() suggestion did not work:

https://lore.kernel.org/lkml/ZvX12_1mK8983cXm@boqun-archlinux/
> 		
> 	
> OTOH if you do:
> 	a1 = a; OPTIMISER_HIDE_VAR(a1);
> 	b1 = b; OPTIMISER_HIDE_VAR(b1);
> 	if (a1 != b1)
> 		return;
> 	// code using a and b
> (which I think is)

This is in line with Linus' suggestion, which is the approach I
retained.

> 
>> 2) Perform the equality check on the versions resulting of hiding
>> both variables, making sure those versions of the variables are
>> not dereferenced afterwards. (as suggested by Linus)
> 
> then the compiler can't possibly reverse the asm blocks.

Indeed.

Thanks,

Mathieu

> 
> 	David
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)

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