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From:   Joel Fernandes <joel@joelfernandes.org>
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Subject: Re: [PATCH 1/2] docs: rcu: Add cautionary note on plain-accesses to requirements
Date:   Thu, 3 Aug 2023 08:35:43 -0400
Message-Id: <148CE02E-BBEC-4D30-9C75-6632A110FFC0@joelfernandes.org>
References: <A272EB21-28F9-4FFE-A3BB-6689807CC3ED@gmail.com>
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>
In-Reply-To: <A272EB21-28F9-4FFE-A3BB-6689807CC3ED@gmail.com>
To:     Alan Huang <mmpgouride@gmail.com>
Precedence: bulk


> 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/Doc=
umentation/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 counti=
ng
>> .. |high-quality implementation of C11 memory_order_consume [PDF]| replac=
e:: 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 optimizati=
ons) 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-determinis=
tic 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 v=
alue
>> +of ``gp`` into a register in line 5, and then using the value of the reg=
ister
>> +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=
.

Yes, a bad outcome is perfectly legal amidst data race. Who said it is not l=
egal?

>=20
> But the example here is different,

That is intentional. Wills discussion partially triggered this. Though I am w=
ondering
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 t=
he address dependency
> doesn=E2=80=99t exist anymore.
>=20
> What am I missing here?

Maybe you are trying to rationalize too much that the sequence mentioned can=
not result
in a counter intuitive outcome like I did?

The point AFAIU is not just about line 10 but that the compiler can replace a=
ny of the
lines after the plain access with the cached value.

Thanks.


>=20
>> +plain accesses of a memory location with rcu_dereference() of the same m=
emory
>> +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
>>=20
>=20