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@xxxxxxxxxxxx> Acked-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxx> Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx> Cc: Will Deacon <will@xxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Boqun Feng <boqun.feng@xxxxxxxxx> Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> Cc: John Stultz <jstultz@xxxxxxxxxx> Cc: Neeraj Upadhyay <Neeraj.Upadhyay@xxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Boqun Feng <boqun.feng@xxxxxxxxx> Cc: Frederic Weisbecker <frederic@xxxxxxxxxx> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx> Cc: Uladzislau Rezki <urezki@xxxxxxxxx> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx> Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx> Cc: Zqiang <qiang.zhang1211@xxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Waiman Long <longman@xxxxxxxxxx> Cc: Mark Rutland <mark.rutland@xxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Vlastimil Babka <vbabka@xxxxxxx> Cc: maged.michael@xxxxxxxxx Cc: Mateusz Guzik <mjguzik@xxxxxxxxx> Cc: Gary Guo <gary@xxxxxxxxxxx> Cc: Jonas Oberhauser <jonas.oberhauser@xxxxxxxxxxxxxxx> Cc: rcu@xxxxxxxxxxxxxxx Cc: linux-mm@xxxxxxxxx Cc: lkmm@xxxxxxxxxxxxxxx Cc: Nikita Popov <github@xxxxxxxxxx> Cc: llvm@xxxxxxxxxxxxxxx --- Changes since v0: - Include feedback from Alan Stern. Changes since v1: - Include feedback from Paul E. McKenney. --- 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 -- 2.39.2
