Re: [PATCH 2/2] Documentation: RCU: Refer to ptr_eq()
On 2024-09-28 16:58, Alan Stern wrote: On Sat, Sep 28, 2024 at 09:51:28AM -0400, Mathieu Desnoyers wrote: [...] -- 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 Note: here I need to update the wording as well: +- 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:: + 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". OK. + 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. OK. + + 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". OK. Will fix in my next round. Thanks, Mathieu 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 -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com
Re: [PATCH 2/2] Documentation: RCU: Refer to ptr_eq()
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 > 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.mich...@gmail.com > Cc: Mateusz Guzik > Cc: Gary Guo > Cc: Jonas Oberhauser > 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 >
[PATCH 2/2] Documentation: RCU: Refer to ptr_eq()
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.mich...@gmail.com Cc: Mateusz Guzik Cc: Gary Guo Cc: Jonas Oberhauser 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 + 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 + 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); + + 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 + 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
