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Sun, 29 Sep 2024 12:11:58 -0400 (EDT) Message-ID: <3a298b64-dd7c-4a0d-950e-8e5b98b39fee@efficios.com> Date: Sun, 29 Sep 2024 12:09:54 -0400 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 2/2] Documentation: RCU: Refer to ptr_eq() To: paulmck@kernel.org Cc: Linus Torvalds , linux-kernel@vger.kernel.org, Greg Kroah-Hartman , Sebastian Andrzej Siewior , Will Deacon , Peter Zijlstra , Boqun Feng , Alan Stern , John Stultz , Neeraj Upadhyay , Frederic Weisbecker , Joel Fernandes , Josh Triplett , Uladzislau Rezki , Steven Rostedt , Lai Jiangshan , Zqiang , Ingo Molnar , Waiman Long , Mark Rutland , Thomas Gleixner , Vlastimil Babka , maged.michael@gmail.com, Mateusz Guzik , Gary Guo , Jonas Oberhauser , rcu@vger.kernel.org, linux-mm@kvack.org, lkmm@lists.linux.dev, Nikita Popov , llvm@lists.linux.dev References: <20240929111608.1016757-1-mathieu.desnoyers@efficios.com> <20240929111608.1016757-3-mathieu.desnoyers@efficios.com> <8153f0f1-fd18-4ae1-9dc5-f9b725429cad@paulmck-laptop> From: Mathieu Desnoyers Content-Language: en-US In-Reply-To: <8153f0f1-fd18-4ae1-9dc5-f9b725429cad@paulmck-laptop> Content-Type: text/plain; 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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 >> 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 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. 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