On Sun, Sep 29, 2024 at 12:09:54PM -0400, Mathieu Desnoyers wrote: > On 2024-09-29 17:51, Paul E. McKenney wrote: > > 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. :-/ > > Changing for the following: > > + 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); > > > > > Other than that, loks good! > > Let me know if I should add an acked-by from you on this > documentation patch as well. Much better! Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > Thanks, > > Mathieu > > > > > 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 > > > > > -- > Mathieu Desnoyers > EfficiOS Inc. > https://www.efficios.com >
