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.desnoyers@xxxxxxxxxxxx> > 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. > --- > 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 >
