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

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Message-ID: <490f7491-5e4f-4c03-89a0-854aff3c7ecf@paulmck-laptop>
Date: Mon, 30 Sep 2024 06:54:22 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
Cc: Alan Stern <stern@...land.harvard.edu>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	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,
	Mateusz Guzik <mjguzik@...il.com>, Gary Guo <gary@...yguo.net>,
	rcu@...r.kernel.org, linux-mm@...ck.org, lkmm@...ts.linux.dev
Subject: Re: [PATCH 1/2] compiler.h: Introduce ptr_eq() to preserve address
 dependency

On Mon, Sep 30, 2024 at 02:06:12PM +0200, Jonas Oberhauser wrote:
> 
> 
> Am 9/30/2024 um 1:04 PM schrieb Paul E. McKenney:
> > On Mon, Sep 30, 2024 at 11:42:11AM +0200, Jonas Oberhauser wrote:
> > > 
> > > 
> > > I also currently don't see any major difference between the constant and
> > > register case. The point is that the address is known before loading into b,
> > > and hence the compiler + hardware can speculatively load *b before loading
> > > into b.
> > 
> > In theory, true.  In practice, in the register case, you need a little
> > more bad luck for the compiler to be able to exploit your mistake.
> 
> If there's one thing I've never run out of, then it is bad luck with
> technology.

Careful!  Someone might try to recruit you as tester.  ;-)

							Thanx, Paul