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

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Message-ID: <973ae617-96a8-456a-a805-af3d61270125@paulmck-laptop>
Date: Mon, 30 Sep 2024 04:04:26 -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 11:42:11AM +0200, Jonas Oberhauser wrote:
> 
> 
> Am 9/28/2024 um 11:15 PM schrieb Alan Stern:
> > On Sat, Sep 28, 2024 at 11:55:22AM -0400, Mathieu Desnoyers wrote:
> > > On 2024-09-28 17:49, Alan Stern wrote:
> > > > Isn't it true that on strongly ordered CPUs, a compiler barrier is
> > > > sufficient to prevent the rcu_dereference() problem?  So the whole idea
> > > > behind ptr_eq() is that it prevents the problem on all CPUs.
> > > 
> > > Correct. But given that we have ptr_eq(), it's good to show how it
> > > equally prevents the compiler from reordering address-dependent loads
> > > (comparison with constant) *and* prevents the compiler from using
> > > one pointer rather than the other (comparison between two non-constant
> > > pointers) which affects speculation on weakly-ordered CPUs.
> > 
> > I don't see how these two things differ from each other.  In the
> > comparison-with-a-constant case, how is the compiler reordering
> > anything?  Isn't it just using the constant address rather than the
> > loaded pointer and thereby breaking the address dependency?
> 
> 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.
> 
> The only difference is how far before loading into b the address is known.

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.

Still, it is indeed far better to attain a state of bliss by keeping
the compiler ignorant.

							Thanx, Paul