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Subject: Re: [PATCH 1/2] docs: rcu: Add cautionary note on plain-accesses to
 requirements
From:   Alan Huang <mmpgouride@gmail.com>
In-Reply-To: <148CE02E-BBEC-4D30-9C75-6632A110FFC0@joelfernandes.org>
Date:   Thu, 3 Aug 2023 21:36:06 +0800
Cc:     linux-kernel@vger.kernel.org, rcu@vger.kernel.org,
        Will Deacon <will@kernel.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Zqiang <qiang.zhang1211@gmail.com>,
        Jonathan Corbet <corbet@lwn.net>
Content-Transfer-Encoding: quoted-printable
Message-Id: <E96297CF-6C81-4BB5-BA5A-6734F6B629F4@gmail.com>
References: <A272EB21-28F9-4FFE-A3BB-6689807CC3ED@gmail.com>
 <148CE02E-BBEC-4D30-9C75-6632A110FFC0@joelfernandes.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Precedence: bulk


> 2023=E5=B9=B48=E6=9C=883=E6=97=A5 =E4=B8=8B=E5=8D=888:35=EF=BC=8CJoel =
Fernandes <joel@joelfernandes.org> =E5=86=99=E9=81=93=EF=BC=9A
>=20
>=20
>=20
>> On Aug 3, 2023, at 8:09 AM, Alan Huang <mmpgouride@gmail.com> wrote:
>>=20
>> =EF=BB=BF
>>> 2023=E5=B9=B48=E6=9C=883=E6=97=A5 11:24=EF=BC=8CJoel Fernandes =
(Google) <joel@joelfernandes.org> =E5=86=99=E9=81=93=EF=BC=9A
>>>=20
>>> 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().
>>>=20
>>> 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().
>>>=20
>>> Link: =
https://lore.kernel.org/all/20230728124412.GA21303@willie-the-truck/
>>> Cc: Will Deacon <will@kernel.org>
>>> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
>>> ---
>>> .../RCU/Design/Requirements/Requirements.rst  | 32 =
+++++++++++++++++++
>>> 1 file changed, 32 insertions(+)
>>>=20
>>> 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
>>>=20
>>> +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 =3D gp; // Plain-load of GP.
>>> +       6   printk("Point gp =3D %p\n", tmp);
>>> +       7
>>> +       8   p =3D 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
>>=20
>> Will=E2=80=99s example is:
>>=20
>>    // Assume *ptr is initially 0 and somebody else writes it to 1
>>    // concurrently
>>=20
>>    foo =3D *ptr;
>>    bar =3D READ_ONCE(*ptr);
>>    baz =3D *ptr;
>>=20
>> Then the compiler is within its right to reorder it to:
>>=20
>>   foo =3D *ptr;
>>   baz =3D *ptr;
>>   bar =3D READ_ONCE(*ptr);
>>=20
>> So, the result foo =3D=3D baz =3D=3D 0 but bar =3D=3D 1 is perfectly =
legal.
>=20
> 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.

>=20
>>=20
>> But the example here is different,
>=20
> That is intentional. Wills discussion partially triggered this. Though =
I am wondering
> if we should document that as well.
>=20
>> 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=E2=80=99t exist anymore.
>>=20
>> What am I missing here?
>=20
> Maybe you are trying to rationalize too much that the sequence =
mentioned cannot result
> in a counter intuitive outcome like I did?
>=20
> 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=E2=80=99s also a data race, right? But the kernel considers it is =
Okay if the write is machine word aligned.

>=20
> Thanks.
>=20
>=20
>=20
>>=20
>>> +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.
>>> --=20
>>> 2.41.0.585.gd2178a4bd4-goog