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

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Message-ID: <b9e83fb9-c413-4b81-be7d-ddd5f670c773@efficios.com>
Date: Mon, 7 Oct 2024 09:21:09 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: David Laight <David.Laight@...LAB.COM>,
 'Jonas Oberhauser' <jonas.oberhauser@...weicloud.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

On 2024-10-07 15:18, David Laight wrote:
> 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.

Yes.

> 
>>> 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.

Yes.

> 
> 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?)

Linus asked that we hide both inputs, otherwise it's really asking
for trouble in terms of API misuse.

Thanks,

Mathieu

> 
> 	David
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)

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