linux-kernel - Re: [RFC PATCH v4 1/4] compiler.h: Introduce ptr

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Message-ID: <cb55e583-395f-4b83-925f-c341c95903c1@efficios.com>
Date: Thu, 18 Dec 2025 08:51:02 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: David Laight <david.laight.linux@...il.com>
Cc: Boqun Feng <boqun.feng@...il.com>, Joel Fernandes
 <joel@...lfernandes.org>, "Paul E. McKenney" <paulmck@...nel.org>,
 linux-kernel@...r.kernel.org, Nicholas Piggin <npiggin@...il.com>,
 Michael Ellerman <mpe@...erman.id.au>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Will Deacon <will@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Alan Stern <stern@...land.harvard.edu>, John Stultz <jstultz@...gle.com>,
 Neeraj Upadhyay <Neeraj.Upadhyay@....com>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Frederic Weisbecker <frederic@...nel.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>,
 Jonas Oberhauser <jonas.oberhauser@...weicloud.com>, rcu@...r.kernel.org,
 linux-mm@...ck.org, lkmm@...ts.linux.dev, Gary Guo <gary@...yguo.net>,
 Nikita Popov <github@...pov.com>, llvm@...ts.linux.dev
Subject: Re: [RFC PATCH v4 1/4] compiler.h: Introduce ptr_eq() to preserve
 address dependency

On 2025-12-18 04:03, David Laight wrote:
[...]
>> + *
>> + * The compiler barrier() is ineffective at fixing this issue. It does
>> + * not prevent the compiler CSE from losing the address dependency:
>> + *
>> + * int fct_2_volatile_barriers(void)
>> + * {
>> + *     int *a, *b;
>> + *
>> + *     do {
>> + *         a = READ_ONCE(p);
>> + *         asm volatile ("" : : : "memory");
>> + *         b = READ_ONCE(p);
>> + *     } while (a != b);
>> + *     asm volatile ("" : : : "memory");  <-- barrier()
>> + *     return *b;
>> + * }
>> + *
>> + * With gcc 14.2 (arm64):
>> + *
>> + * fct_2_volatile_barriers:
>> + *         adrp    x0, .LANCHOR0
>> + *         add     x0, x0, :lo12:.LANCHOR0
>> + * .L2:
>> + *         ldr     x1, [x0]  <-- x1 populated by first load.
>> + *         ldr     x2, [x0]
>> + *         cmp     x1, x2
>> + *         bne     .L2
>> + *         ldr     w0, [x1]  <-- x1 is used for access which should depend on b.
>> + *         ret
>> + *
>> + * On weakly-ordered architectures, this lets CPU speculation use the
>> + * result from the first load to speculate "ldr w0, [x1]" before
>> + * "ldr x2, [x0]".
>> + * Based on the RCU documentation, the control dependency does not
>> + * prevent the CPU from speculating loads.
> 
> I'm not sure that example (of something that doesn't work) is really necessary.
> The simple example of, given:
> 	return a == b ? *a : 0;
> the generated code might speculatively dereference 'b' (not a) before returning
> zero when the pointers are different.

In the past discussion that led to this new API, AFAIU, Linus made it
clear that this counter example needs to be in a comment:

https://lore.kernel.org/lkml/CAHk-=wgBgh5U+dyNaN=+XCdcm2OmgSRbcH4Vbtk8i5ZDGwStSA@mail.gmail.com/

This counter-example is what convinced him that this addresses a real
issue.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com