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

[Joel Fernandes]

> On Aug 3, 2023, at 8:01 PM, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> 
> On Fri, Aug 04, 2023 at 03:25:57AM +0800, Alan Huang 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.
>>> 
>>>>>> 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.
> 
> Volatile accesses are not specified very well in the standard.  However,
> as a practical matter, compilers that wish to be able to device drivers
> (whether in kernels or userspace applications) must compile those volatile
> accesses in such a way to allow reliable device drivers to be written.

Agreed.

> 
>> However, the kernel consider the volatile access to be atomic, right?
> 
> The compiler must therefore act as if a volatile access to an aligned
> machine-word size location is atomic.  To see this, consider accesses
> to memory that is shared by a device driver and that device's firmware,
> both of which are written in either C or C++.

Btw it appears TSAN complaints bitterly on even volatile 4 byte data races.
Hence we have to explicitly use atomic API for data race accesses in Chrome.

Thanks,
Joel



> Does that help?
> 
>                            Thanx, Paul
> 
>> 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.
>> 
>>> 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
>> 
>>