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

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Message-ID: <43788527053542e78001820857445e4d@AcuMS.aculab.com>
Date: Mon, 7 Oct 2024 13:18:58 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Jonas Oberhauser' <jonas.oberhauser@...weicloud.com>, Mathieu Desnoyers
	<mathieu.desnoyers@...icios.com>, 'Alan Stern' <stern@...land.harvard.edu>
CC: 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

From: Jonas Oberhauser
> Sent: 07 October 2024 12:55
> 
> Am 10/3/2024 um 3:23 PM schrieb Mathieu Desnoyers:
> > 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)
> 
> This should not be guaranteed to work, because right after the
> comparison the compiler can do b=a, then it doesn't matter how much you
> hide afterwards.
> 
> However it might work if you escape the addresses of a and b first, in
> which case the compiler will not do b=a anymore, but it might force the
> compiler to put a and b on the stack, which has some performance impact.

Nope, as pointed out last week, the compiler can move the 'a == b'
check to before the OPTIMISER_HID_VAR() and then use the same register
for both of them.

> > 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)

That (and other things) could usefully use:
#define OPTIMISER_HIDE_VALUE(x) \
	({ __auto_type _x = x; OPTIMISER_HIDE_VAR(_x); _x; })
You'll almost certainly end up with a register-register move
even if 'x' isn't used afterwards.

The calling could just become:
	if (a == OPTIMISER_HIDE_VALUE(b) ...
since it is likely that you only care about one of the pointers.
(Actually isn't hiding one of them always enough?)

	David

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