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 <mathieu.desnoy...@efficios.com> > Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org> > Cc: Sebastian Andrzej Siewior <bige...@linutronix.de> > Cc: "Paul E. McKenney" <paul...@kernel.org> > Cc: Will Deacon <w...@kernel.org> > Cc: Peter Zijlstra <pet...@infradead.org> > Cc: Boqun Feng <boqun.f...@gmail.com> > Cc: Alan Stern <st...@rowland.harvard.edu> > Cc: John Stultz <jstu...@google.com> > Cc: Neeraj Upadhyay <neeraj.upadh...@amd.com> > Cc: Linus Torvalds <torva...@linux-foundation.org> > Cc: Boqun Feng <boqun.f...@gmail.com> > Cc: Frederic Weisbecker <frede...@kernel.org> > Cc: Joel Fernandes <j...@joelfernandes.org> > Cc: Josh Triplett <j...@joshtriplett.org> > Cc: Uladzislau Rezki <ure...@gmail.com> > Cc: Steven Rostedt <rost...@goodmis.org> > Cc: Lai Jiangshan <jiangshan...@gmail.com> > Cc: Zqiang <qiang.zhang1...@gmail.com> > Cc: Ingo Molnar <mi...@redhat.com> > Cc: Waiman Long <long...@redhat.com> > Cc: Mark Rutland <mark.rutl...@arm.com> > Cc: Thomas Gleixner <t...@linutronix.de> > Cc: Vlastimil Babka <vba...@suse.cz> > Cc: maged.mich...@gmail.com > Cc: Mateusz Guzik <mjgu...@gmail.com> > Cc: Gary Guo <g...@garyguo.net> > Cc: Jonas Oberhauser <jonas.oberhau...@huaweicloud.com> > 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 >
