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
> Cc: Greg Kroah-Hartman
> Cc: Sebastian Andrzej Siewior
> Cc: "Paul E. McKenney"
> Cc: Will Deacon
> Cc: Peter Zijlstra
> Cc: Boqun Feng
> Cc: Alan Stern
> Cc: John Stultz
> Cc: Neeraj Upadhyay
> Cc: Linus Torvalds
> Cc: Boqun Feng
> Cc: Frederic Weisbecker
> Cc: Joel Fernandes
> Cc: Josh Triplett
> Cc: Uladzislau Rezki
> Cc: Steven Rostedt
> Cc: Lai Jiangshan
> Cc: Zqiang
> Cc: Ingo Molnar
> Cc: Waiman Long
> Cc: Mark Rutland
> Cc: Thomas Gleixner
> Cc: Vlastimil Babka
> Cc: maged.mich...@gmail.com
> Cc: Mateusz Guzik
> Cc: Gary Guo
> Cc: Jonas Oberhauser
> Cc: r...@vger.kernel.org
> Cc: linux...@kvack.org
> Cc: l...@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
>
