linux-kernel - Re: [PATCH 1/2] docs: rcu: Add cautionary note on plain-accesses to requirements

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Date:   Fri, 4 Aug 2023 23:47:07 +0800
From:   Alan Huang <mmpgouride@...il.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
        Will Deacon <will@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Zqiang <qiang.zhang1211@...il.com>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH 1/2] docs: rcu: Add cautionary note on plain-accesses to
 requirements

> 
>>> 
>>>>>> But the example here is different,
>>>>> 
>>>>> That is intentional. Wills discussion partially triggered this. Though I am wondering
>>>>> if we should document that as well.
>>>>> 
>>>>>> the compiler can not use the value loaded from line 5
>>>>>> unless the compiler can deduce that the tmp is equals to p in which case the address dependency
>>>>>> doesn’t exist anymore.
>>>>>> 
>>>>>> What am I missing here?
>>>>> 
>>>>> Maybe you are trying to rationalize too much that the sequence mentioned cannot result
>>>>> in a counter intuitive outcome like I did?
>>>>> 
>>>>> The point AFAIU is not just about line 10 but that the compiler can replace any of the
>>>>> lines after the plain access with the cached value.
>>>> 
>>>> Well, IIUC, according to the C standard, the compiler can do anything if there is a data race (undefined behavior).
>>>> 
>>>> However, what if a write is not protected with WRITE_ONCE and the read is marked with READ_ONCE?
>>>> That’s also a data race, right? But the kernel considers it is Okay if the write is machine word aligned.
>>> 
>>> Yes, but there is a compiler between the HLL code and what the
>>> processor sees which can tear the write. How can not using
>>> WRITE_ONCE() prevent store-tearing? See [1]. My understanding is that
>>> it is OK only if the reader did a NULL check. In that case the torn
>> 
>> Yes, a write-write data race where the value is the same is also fine.
>> 
>> But they are still data race, if the compiler is within its right to do anything it likes (due to data race),
>> we still need WRITE_ONCE() in these cases, though it’s semantically safe.
>> 
>> IIUC, even with _ONCE(), the compiler is within its right do anything according to the standard (at least before the upcoming C23), because the standard doesn’t consider a volatile access to be atomic.
>> 
>> However, the kernel consider the volatile access to be atomic, right?
>> 
>> BTW, line 5 in the example is likely to be optimized away. And yes, the compiler can cache the value loaded from line 5 from the perspective of undefined behavior, even if I believe it would be a compiler bug from the perspective of kernel.
> 
> I am actually a bit lost with what you are trying to say.  Are you saying that mixing
> plain accesses with marked accesses is an acceptable practice? 


I’m trying to say that sometimes data race is fine, that’s why we have the data_race().

Even if the standard says data race results in UB.

And IMHO, the possible data race at line 5 in this example is also fine, unless the compiler 
deduces that the value of gp is always the same.


> 
> I would like others to weight in as well since I am not seeing what Alan is suggesting.
> AFAICS, in the absence of barrier(), any optimization caused by plain access
> makes it a bad practice to mix it.
> 
> Thanks,
> 
> - Joel
> 
> 
> 
>> 
>>> result will not change the semantics of the program. But otherwise,
>>> that's bad.
>>> 
>>> [1] https://lwn.net/Articles/793253/#Store%20Tearing
>>> 
>>> thanks,
>>> 
>>> - Joel
>>> 
>>> 
>>>> 
>>>>> 
>>>>> Thanks.
>>>>> 
>>>>> 
>>>>> 
>>>>>> 
>>>>>>> +plain accesses of a memory location with rcu_dereference() of the same memory
>>>>>>> +location, in code involved in a data race.
>>>>>>> +
>>>>>>> In short, updaters use rcu_assign_pointer() and readers use
>>>>>>> rcu_dereference(), and these two RCU API elements work together to
>>>>>>> ensure that readers have a consistent view of newly added data elements.
>>>>>>> --
>>>>>>> 2.41.0.585.gd2178a4bd4-goog