> 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 > >
