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 Acked-by: Alan Stern Acked-by: Paul E. McKenney Reviewed-by: Joel Fernandes (Google) 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.michael@gmail.com Cc: Mateusz Guzik Cc: Gary Guo Cc: Jonas Oberhauser Cc: rcu@vger.kernel.org Cc: linux-mm@kvack.org Cc: lkmm@lists.linux.dev Cc: Nikita Popov Cc: llvm@lists.linux.dev --- Changes since v1: - Include feedback from Paul E. McKenney. Changes since v0: - Include feedback from Alan Stern. --- Documentation/RCU/rcu_dereference.rst | 38 +++++++++++++++++++++++---- 1 file changed, 33 insertions(+), 5 deletions(-) diff --git a/Documentation/RCU/rcu_dereference.rst b/Documentation/RCU/rcu_dereference.rst index 2524dcdadde2..de6175bf430f 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 +126,29 @@ 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) /* BUGGY!!! */ + 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. + Corrected code:: + + p1 = READ_ONCE(gp); + p2 = rcu_dereference(gp); + if (ptr_eq(p1, p2)) + do_default(p2->a); However, comparisons are OK in the following cases: @@ -204,6 +228,10 @@ 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 -- 2.39.5