Re: [PATCH 2/2] Documentation: RCU: Refer to ptr_eq()

Alan Stern
2024-09-28T14:58:38Z

On Sat, Sep 28, 2024 at 09:51:28AM -0400, Mathieu Desnoyers wrote:
> Refer to ptr_eq() in the rcu_dereference() documentation.
>
> ptr_eq() is a mechanism that preserves address dependencies when
> comparing pointers, and should be favored when comparing a pointer
> obtained from rcu_dereference() against another pointer.
>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Will Deacon <will@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: Alan Stern <stern@rowland.harvard.edu>
> Cc: John Stultz <jstultz@google.com>
> Cc: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Cc: Joel Fernandes <joel@joelfernandes.org>
> Cc: Josh Triplett <josh@joshtriplett.org>
> Cc: Uladzislau Rezki <urezki@gmail.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> Cc: Zqiang <qiang.zhang1211@gmail.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Waiman Long <longman@redhat.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: maged.michael@gmail.com
> Cc: Mateusz Guzik <mjguzik@gmail.com>
> Cc: Gary Guo <gary@garyguo.net>
> Cc: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
> Cc: rcu@vger.kernel.org
> Cc: linux-mm@kvack.org
> Cc: lkmm@lists.linux.dev
> ---
> Documentation/RCU/rcu_dereference.rst | 34 +++++++++++++++++++++++----
> 1 file changed, 29 insertions(+), 5 deletions(-)
>
> diff --git a/Documentation/RCU/rcu_dereference.rst b/Documentation/RCU/rcu_dereference.rst
> index 2524dcdadde2..c36b8d1721f6 100644
> --- a/Documentation/RCU/rcu_dereference.rst
> +++ b/Documentation/RCU/rcu_dereference.rst
> @@ -104,11 +104,13 @@ readers working properly:
> after such branches, but can speculate loads, which can again
> result in misordering bugs.
>
> -- Be very careful about comparing pointers obtained from
> - rcu_dereference() against non-NULL values. As Linus Torvalds
> - explained, if the two pointers are equal, the compiler could
> - substitute the pointer you are comparing against for the pointer
> - obtained from rcu_dereference(). For example::
> +- Use relational operators which preserve address dependencies
> + (such as "ptr_eq()") to compare pointers obtained from

Nit: ptr_eq() is an inline function, not a relational operator. Say
"operations that" instead of "relational operators which".

> + rcu_dereference() against non-NULL values or against pointers
> + obtained from prior loads. As Linus Torvalds explained, if the
> + two pointers are equal, the compiler could substitute the
> + pointer you are comparing against for the pointer obtained from
> + rcu_dereference(). For example::
>
> p = rcu_dereference(gp);
> if (p == &default_struct)
> @@ -125,6 +127,23 @@ readers working properly:
> On ARM and Power hardware, the load from "default_struct.a"
> can now be speculated, such that it might happen before the
> rcu_dereference(). This could result in bugs due to misordering.
> + Performing the comparison with "ptr_eq()" ensures the compiler
> + does not perform such transformation.
> +
> + If the comparison is against a pointer obtained from prior
> + loads, the compiler is allowed to use either register for the

This is true even when the comparison is against a pointer obtained from
a later load. Just say "another pointer" instead of "a pointer obtained
from prior loads". (And why would someone need multiple loads to
obtain a single pointer?)

Also, say "pointer" instead of "register".

> + following accesses, which loses the address dependency and
> + allows weakly-ordered architectures such as ARM and PowerPC
> + to speculate the address-dependent load before rcu_dereference().
> + For example::
> +
> + p1 = READ_ONCE(gp);
> + p2 = rcu_dereference(gp);
> + if (p1 == p2)
> + do_default(p2->a);

Here you should say that the compiler could use p1->a rather than p2->a,
destroying the address dependency. That's the whole point of this; you
shouldn't skip over it.

> +
> + Performing the comparison with "ptr_eq()" ensures the compiler
> + preserves the address dependencies.
>
> However, comparisons are OK in the following cases:
>
> @@ -204,6 +223,11 @@ readers working properly:
> comparison will provide exactly the information that the
> compiler needs to deduce the value of the pointer.
>
> + When in doubt, use relational operators that preserve address

Again, "operations" instead of "relational operators".

Alan Stern

> + dependencies (such as "ptr_eq()") to compare pointers obtained
> + from rcu_dereference() against non-NULL values or against
> + pointers obtained from prior loads.
> +
> - Disable any value-speculation optimizations that your compiler
> might provide, especially if you are making use of feedback-based
> optimizations that take data collected from prior runs. Such
> --
> 2.39.2
>