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

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Message-ID: <f635f9ce-fef4-4a9e-bee1-70dbc24a82ad@huaweicloud.com>
Date: Mon, 30 Sep 2024 11:42:11 +0200
From: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
To: Alan Stern <stern@...land.harvard.edu>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.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,
 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

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.

Best wishes,
   jonas