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

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Message-ID: <498f11de5a024f0ca0b70aba4e28b17b@AcuMS.aculab.com>
Date: Thu, 3 Oct 2024 17:07:03 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Mathieu Desnoyers' <mathieu.desnoyers@...icios.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

...
> 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.
		
	
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)

> 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.

	David

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