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

[Joel Fernandes]

> On Aug 3, 2023, at 3:26 PM, Alan Huang <mmpgouride@xxxxxxxxx> wrote:
> 
> 
>> 2023年8月4日 00:01，Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> 写道：
>> 
>>> On Thu, Aug 3, 2023 at 9:36 AM Alan Huang <mmpgouride@xxxxxxxxx> wrote:
>>> 
>>> 
>>>> 2023年8月3日 下午8:35，Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> 写道：
>>>> 
>>>> 
>>>> 
>>>>> On Aug 3, 2023, at 8:09 AM, Alan Huang <mmpgouride@xxxxxxxxx> wrote:
>>>>> 
>>>>> 
>>>>>> 2023年8月3日 11:24，Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> 写道：
>>>>>> 
>>>>>> Add a detailed note to explain the potential side effects of
>>>>>> plain-accessing the gp pointer using a plain load, without using the
>>>>>> rcu_dereference() macros; which might trip neighboring code that does
>>>>>> use rcu_dereference().
>>>>>> 
>>>>>> I haven't verified this with a compiler, but this is what I gather from
>>>>>> the below link using Will's experience with READ_ONCE().
>>>>>> 
>>>>>> Link: https://lore.kernel.org/all/20230728124412.GA21303@willie-the-truck/
>>>>>> Cc: Will Deacon <will@xxxxxxxxxx>
>>>>>> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
>>>>>> ---
>>>>>> .../RCU/Design/Requirements/Requirements.rst  | 32 +++++++++++++++++++
>>>>>> 1 file changed, 32 insertions(+)
>>>>>> 
>>>>>> diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst
>>>>>> index f3b605285a87..e0b896d3fb9b 100644
>>>>>> --- a/Documentation/RCU/Design/Requirements/Requirements.rst
>>>>>> +++ b/Documentation/RCU/Design/Requirements/Requirements.rst
>>>>>> @@ -376,6 +376,38 @@ mechanism, most commonly locking or reference counting
>>>>>> .. |high-quality implementation of C11 memory_order_consume [PDF]| replace:: high-quality implementation of C11 ``memory_order_consume`` [PDF]
>>>>>> .. _high-quality implementation of C11 memory_order_consume [PDF]: http://www.rdrop.com/users/paulmck/RCU/consume.2015.07.13a.pdf
>>>>>> 
>>>>>> +Note that, there can be strange side effects (due to compiler optimizations) if
>>>>>> +``gp`` is ever accessed using a plain load (i.e. without ``READ_ONCE()`` or
>>>>>> +``rcu_dereference()``) potentially hurting any succeeding
>>>>>> +``rcu_dereference()``. For example, consider the code:
>>>>>> +
>>>>>> +   ::
>>>>>> +
>>>>>> +       1 bool do_something_gp(void)
>>>>>> +       2 {
>>>>>> +       3   void *tmp;
>>>>>> +       4   rcu_read_lock();
>>>>>> +       5   tmp = gp; // Plain-load of GP.
>>>>>> +       6   printk("Point gp = %p\n", tmp);
>>>>>> +       7
>>>>>> +       8   p = rcu_dereference(gp);
>>>>>> +       9   if (p) {
>>>>>> +      10     do_something(p->a, p->b);
>>>>>> +      11     rcu_read_unlock();
>>>>>> +      12     return true;
>>>>>> +      13   }
>>>>>> +      14   rcu_read_unlock();
>>>>>> +      15   return false;
>>>>>> +      16 }
>>>>>> +
>>>>>> +The behavior of plain accesses involved in a data race is non-deterministic in
>>>>>> +the face of compiler optimizations. Since accesses to the ``gp`` pointer is
>>>>>> +by-design a data race, the compiler could trip this code by caching the value
>>>>>> +of ``gp`` into a register in line 5, and then using the value of the register
>>>>>> +to satisfy the load in line 10. Thus it is important to never mix
>>>>> 
>>>>> Will’s example is:
>>>>> 
>>>>>  // Assume *ptr is initially 0 and somebody else writes it to 1
>>>>>  // concurrently
>>>>> 
>>>>>  foo = *ptr;
>>>>>  bar = READ_ONCE(*ptr);
>>>>>  baz = *ptr;
>>>>> 
>>>>> Then the compiler is within its right to reorder it to:
>>>>> 
>>>>> foo = *ptr;
>>>>> baz = *ptr;
>>>>> bar = READ_ONCE(*ptr);
>>>>> 
>>>>> So, the result foo == baz == 0 but bar == 1 is perfectly legal.
>>>> 
>>>> Yes, a bad outcome is perfectly legal amidst data race. Who said it is not legal?
>>> 
>>> My understanding is that it is legal even without data race, and the compiler only keeps the order of volatile access.
>> 
>> Yes, but I can bet on it the author of the code would not have
>> intended such an outcome, if they did then Will wouldn't have been
>> debugging it ;-). That's why I called it a bad outcome. The goal of
>> this patch is to document such a possible unintentional outcome.

Please trim replies if possible.

>> 
>>>>> 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 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
> 
>