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

[Joel Fernandes]

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
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
>