On Sun, Sep 29, 2024 at 07:16:08AM -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.desnoy...@efficios.com>
> Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org>
> Cc: Sebastian Andrzej Siewior <bige...@linutronix.de>
> Cc: "Paul E. McKenney" <paul...@kernel.org>
> Cc: Will Deacon <w...@kernel.org>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Cc: Boqun Feng <boqun.f...@gmail.com>
> Cc: Alan Stern <st...@rowland.harvard.edu>
> Cc: John Stultz <jstu...@google.com>
> Cc: Neeraj Upadhyay <neeraj.upadh...@amd.com>
> Cc: Linus Torvalds <torva...@linux-foundation.org>
> Cc: Boqun Feng <boqun.f...@gmail.com>
> Cc: Frederic Weisbecker <frede...@kernel.org>
> Cc: Joel Fernandes <j...@joelfernandes.org>
> Cc: Josh Triplett <j...@joshtriplett.org>
> Cc: Uladzislau Rezki <ure...@gmail.com>
> Cc: Steven Rostedt <rost...@goodmis.org>
> Cc: Lai Jiangshan <jiangshan...@gmail.com>
> Cc: Zqiang <qiang.zhang1...@gmail.com>
> Cc: Ingo Molnar <mi...@redhat.com>
> Cc: Waiman Long <long...@redhat.com>
> Cc: Mark Rutland <mark.rutl...@arm.com>
> Cc: Thomas Gleixner <t...@linutronix.de>
> Cc: Vlastimil Babka <vba...@suse.cz>
> Cc: maged.mich...@gmail.com
> Cc: Mateusz Guzik <mjgu...@gmail.com>
> Cc: Gary Guo <g...@garyguo.net>
> Cc: Jonas Oberhauser <jonas.oberhau...@huaweicloud.com>
> Cc: r...@vger.kernel.org
> Cc: linux...@kvack.org
> Cc: l...@lists.linux.dev
> Cc: Nikita Popov <git...@npopov.com>
> Cc: l...@lists.linux.dev
> ---
> Changes since v0:
> - Include feedback from Alan Stern.
> ---
> Documentation/RCU/rcu_dereference.rst | 32 ++++++++++++++++++++++-----
> 1 file changed, 27 insertions(+), 5 deletions(-)
>
> diff --git a/Documentation/RCU/rcu_dereference.rst
> b/Documentation/RCU/rcu_dereference.rst
> index 2524dcdadde2..9ef97b7ca74d 100644
> --- a/Documentation/RCU/rcu_dereference.rst
> +++ b/Documentation/RCU/rcu_dereference.rst
> @@ -104,11 +104,12 @@ 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 operations that preserve address dependencies (such as
> + "ptr_eq()") to compare pointers obtained from rcu_dereference()
> + against non-NULL pointers. 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 +126,23 @@ 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 another pointer, the compiler is
> + allowed to use either pointer for the 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);
> +
> + The compiler can use p1->a rather than p2->a, destroying the
> + address dependency. Performing the comparison with "ptr_eq()"
> + ensures the compiler preserves the address dependencies.
Bitter experience leads me to suggest a "// BUGGY" comment on the "if"
statement in the above example, and a corrected code snippet right here. :-/
Other than that, loks good!
Thanx, Paul
> However, comparisons are OK in the following cases:
>
> @@ -204,6 +222,10 @@ readers working properly:
> comparison will provide exactly the information that the
> compiler needs to deduce the value of the pointer.
>
> + When in doubt, use operations that preserve address dependencies
> + (such as "ptr_eq()") to compare pointers obtained from
> + rcu_dereference() against non-NULL pointers.
> +
> - 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
>
