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. > 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++. 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 > >
