linux-kernel - Re: [PATCH memory-model] docs: memory-barriers: Add note on compiler transformation and address deps

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Date:   Wed, 18 Oct 2023 12:11:58 +0200
From:   Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
To:     paulmck@...nel.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-doc@...r.kernel.org
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH memory-model] docs: memory-barriers: Add note on compiler
 transformation and address deps

Hi Paul,

on a second thought. Why can't the compiler always do, e.g.,

     int *p = READ_ONCE(shared_ptr);

     assert (*p == 0);

~>

     int *p = READ_ONCE(shared_ptr);

     int val = x; // x is some object that definitely won't segfault, 
but may very well be owned by another thread right now
     if (p != &x) val = *p;

     assert (val == 0);

and in case p == &x, the address dependency is elided


Best wishes,

jonas

Am 10/6/2023 um 6:39 PM schrieb Jonas Oberhauser:
> Hi Paul,
>
> The "more up-to-date information" makes it sound like (some of) the 
> information in this section is out-of-date/no longer valid.
>
> But after reading the sections, it seems the information is valid, but 
> discusses mostly the history of address dependency barriers.
>
> Given that the sepcond part  specifically already starts with a 
> disclaimer that this information is purely relevant to people 
> interested in history or working on alpha, I think it would make more 
> sense to modify things slightly differently.
>
> Firstly I'd remove the "historical" part in the first section, and add 
> two short paragraphs explaining that
>
> - every marked access implies a address dependency barrier
>
> - address dependencies considered by the model are *semantic* 
> dependencies, meaning that a *syntactic* dependency is not sufficient 
> to imply ordering; see the rcu file for some examples where compilers 
> can elide syntactic dependencies
>
> Secondly, I'd not add the disclaimer to the second section; there's 
> already a link to rcu_dereference in that section ( 
> https://github.com/torvalds/linux/blob/master/Documentation/memory-barriers.txt#L634 
> ), and already a small text explaining that the section is historical.
>
>
> Best wishes,
>
> jonas
>
>
> Am 10/5/2023 um 6:53 PM schrieb Paul E. McKenney:
>> The compiler has the ability to cause misordering by destroying
>> address-dependency barriers if comparison operations are used. Add a
>> note about this to memory-barriers.txt in the beginning of both the
>> historical address-dependency sections and point to rcu-dereference.rst
>> for more information.
>>
>> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
>> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
>>
>> diff --git a/Documentation/memory-barriers.txt 
>> b/Documentation/memory-barriers.txt
>> index 06e14efd8662..d414e145f912 100644
>> --- a/Documentation/memory-barriers.txt
>> +++ b/Documentation/memory-barriers.txt
>> @@ -396,6 +396,10 @@ Memory barriers come in four basic varieties:
>>        (2) Address-dependency barriers (historical).
>> +     [!] This section is marked as HISTORICAL: For more up-to-date
>> +     information, including how compiler transformations related to 
>> pointer
>> +     comparisons can sometimes cause problems, see
>> +     Documentation/RCU/rcu_dereference.rst.
>>          An address-dependency barrier is a weaker form of read 
>> barrier.  In the
>>        case where two loads are performed such that the second 
>> depends on the
>> @@ -556,6 +560,9 @@ There are certain things that the Linux kernel 
>> memory barriers do not guarantee:
>>     ADDRESS-DEPENDENCY BARRIERS (HISTORICAL)
>>   ----------------------------------------
>> +[!] This section is marked as HISTORICAL: For more up-to-date 
>> information,
>> +including how compiler transformations related to pointer 
>> comparisons can
>> +sometimes cause problems, see Documentation/RCU/rcu_dereference.rst.
>>     As of v4.15 of the Linux kernel, an smp_mb() was added to 
>> READ_ONCE() for
>>   DEC Alpha, which means that about the only people who need to pay 
>> attention