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

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Message-Id: <E2551572-494D-46A0-A1D5-DF91DDB8B9F9@gmail.com>
Date: Mon, 30 Sep 2024 17:27:45 +0800
From: Alan Huang <mmpgouride@...il.com>
To: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Alan Stern <stern@...land.harvard.edu>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 LKML <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 (Sony)" <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 <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

2024年9月30日 17:15，Alan Huang <mmpgouride@...il.com> 写道：
> 
> 2024年9月30日 16:57，Jonas Oberhauser <jonas.oberhauser@...weicloud.com> 写道：
>> 
>> 
>> 
>> Am 9/29/2024 um 12:26 AM schrieb Alan Huang:
>>> 2024年9月28日 23:55，Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote：
>>>> 
>>>> On 2024-09-28 17:49, Alan Stern wrote:
>>>>> On Sat, Sep 28, 2024 at 11:32:18AM -0400, Mathieu Desnoyers wrote:
>>>>>> On 2024-09-28 16:49, Alan Stern wrote:
>>>>>>> On Sat, Sep 28, 2024 at 09:51:27AM -0400, Mathieu Desnoyers wrote:
>>>>>>>> equality, which does not preserve address dependencies and allows the
>>>>>>>> following misordering speculations:
>>>>>>>> 
>>>>>>>> - If @b is a constant, the compiler can issue the loads which depend
>>>>>>>>   on @a before loading @a.
>>>>>>>> - If @b is a register populated by a prior load, weakly-ordered
>>>>>>>>   CPUs can speculate loads which depend on @a before loading @a.
>>>>>>> 
>>>>>>> It shouldn't matter whether @a and @b are constants, registers, or
>>>>>>> anything else.  All that matters is that the compiler uses the wrong
>>>>>>> one, which allows weakly ordered CPUs to speculate loads you wouldn't
>>>>>>> expect it to, based on the source code alone.
>>>>>> 
>>>>>> I only partially agree here.
>>>>>> 
>>>>>> On weakly-ordered architectures, indeed we don't care whether the
>>>>>> issue is caused by the compiler reordering the code (constant)
>>>>>> or the CPU speculating the load (registers).
>>>>>> 
>>>>>> However, on strongly-ordered architectures, AFAIU, only the constant
>>>>>> case is problematic (compiler reordering the dependent load), because
>>>>> I thought you were trying to prevent the compiler from using one pointer
>>>>> instead of the other, not trying to prevent it from reordering anything.
>>>>> Isn't this the point the documentation wants to get across when it says
>>>>> that comparing pointers can be dangerous?
>>>> 
>>>> The motivation for introducing ptr_eq() is indeed because the
>>>> compiler barrier is not sufficient to prevent the compiler from
>>>> using one pointer instead of the other.
>>> barrier_data(&b) prevents that.
>> 
>> I don't think one barrier_data can garantuee preventing this, because right after doing the comparison, the compiler still could do b=a.
>> 
>> In that case you would be guaranteed to use the value in b, but that value is not the value loaded into b originally but rather the value loaded into a, and hence your address dependency goes to the wrong load still.
> 
> After barrier_data(&b), *b will be loaded from memory, you mean even if *b is loaded from memory, the address dependency goes to the wrong load still?

Sorry, *b should b.

> 
>> 
>> However, doing
>> 
>> barrier_data(&b);
>> if (a == b) {
>>  barrier();
>>  foo(*b);
>> }
>> 
>> might maybe prevent it, because after the address of b is escaped, the compiler might no longer be allowed to just do b=a;, but I'm not sure if that is completely correct, since the compiler knows b==a and no other thread can be concurrently modifying a or b. Therefore, given that the compiler knows the hardware, it might know that assigning b=a would not cause any  race-related issues even if another thread was reading b concurrently.
>> 
>> Finally, it may be only a combination of barrier_data and making b volatile could be guaranteed to solve the issue, but the code will be very obscure compared to using ptr_eq.
>> 
>> jonas