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@efficios.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Will Deacon <will@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: John Stultz <jstultz@google.com>
Cc: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Zqiang <qiang.zhang1211@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Waiman Long <longman@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: maged.michael@gmail.com
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Gary Guo <gary@garyguo.net>
Cc: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
Cc: rcu@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: lkmm@lists.linux.dev
Cc: Nikita Popov <github@npopov.com>
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.
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
--
2.39.2On 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@efficios.com> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> > Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de> > Cc: "Paul E. McKenney" <paulmck@kernel.org> > Cc: Will Deacon <will@kernel.org> > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Boqun Feng <boqun.feng@gmail.com> > Cc: Alan Stern <stern@rowland.harvard.edu> > Cc: John Stultz <jstultz@google.com> > Cc: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com> > Cc: Linus Torvalds <torvalds@linux-foundation.org> > Cc: Boqun Feng <boqun.feng@gmail.com> > Cc: Frederic Weisbecker <frederic@kernel.org> > Cc: Joel Fernandes <joel@joelfernandes.org> > Cc: Josh Triplett <josh@joshtriplett.org> > Cc: Uladzislau Rezki <urezki@gmail.com> > Cc: Steven Rostedt <rostedt@goodmis.org> > Cc: Lai Jiangshan <jiangshanlai@gmail.com> > Cc: Zqiang <qiang.zhang1211@gmail.com> > Cc: Ingo Molnar <mingo@redhat.com> > Cc: Waiman Long <longman@redhat.com> > Cc: Mark Rutland <mark.rutland@arm.com> > Cc: Thomas Gleixner <tglx@linutronix.de> > Cc: Vlastimil Babka <vbabka@suse.cz> > Cc: maged.michael@gmail.com > Cc: Mateusz Guzik <mjguzik@gmail.com> > Cc: Gary Guo <gary@garyguo.net> > Cc: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com> > Cc: rcu@vger.kernel.org > Cc: linux-mm@kvack.org > Cc: lkmm@lists.linux.dev > Cc: Nikita Popov <github@npopov.com> > 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. :-/ 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 >
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@efficios.com> >> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> >> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de> >> Cc: "Paul E. McKenney" <paulmck@kernel.org> >> Cc: Will Deacon <will@kernel.org> >> Cc: Peter Zijlstra <peterz@infradead.org> >> Cc: Boqun Feng <boqun.feng@gmail.com> >> Cc: Alan Stern <stern@rowland.harvard.edu> >> Cc: John Stultz <jstultz@google.com> >> Cc: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com> >> Cc: Linus Torvalds <torvalds@linux-foundation.org> >> Cc: Boqun Feng <boqun.feng@gmail.com> >> Cc: Frederic Weisbecker <frederic@kernel.org> >> Cc: Joel Fernandes <joel@joelfernandes.org> >> Cc: Josh Triplett <josh@joshtriplett.org> >> Cc: Uladzislau Rezki <urezki@gmail.com> >> Cc: Steven Rostedt <rostedt@goodmis.org> >> Cc: Lai Jiangshan <jiangshanlai@gmail.com> >> Cc: Zqiang <qiang.zhang1211@gmail.com> >> Cc: Ingo Molnar <mingo@redhat.com> >> Cc: Waiman Long <longman@redhat.com> >> Cc: Mark Rutland <mark.rutland@arm.com> >> Cc: Thomas Gleixner <tglx@linutronix.de> >> Cc: Vlastimil Babka <vbabka@suse.cz> >> Cc: maged.michael@gmail.com >> Cc: Mateusz Guzik <mjguzik@gmail.com> >> Cc: Gary Guo <gary@garyguo.net> >> Cc: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com> >> Cc: rcu@vger.kernel.org >> Cc: linux-mm@kvack.org >> Cc: lkmm@lists.linux.dev >> Cc: Nikita Popov <github@npopov.com> >> 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
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@efficios.com> > > > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> > > > Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de> > > > Cc: "Paul E. McKenney" <paulmck@kernel.org> > > > Cc: Will Deacon <will@kernel.org> > > > Cc: Peter Zijlstra <peterz@infradead.org> > > > Cc: Boqun Feng <boqun.feng@gmail.com> > > > Cc: Alan Stern <stern@rowland.harvard.edu> > > > Cc: John Stultz <jstultz@google.com> > > > Cc: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com> > > > Cc: Linus Torvalds <torvalds@linux-foundation.org> > > > Cc: Boqun Feng <boqun.feng@gmail.com> > > > Cc: Frederic Weisbecker <frederic@kernel.org> > > > Cc: Joel Fernandes <joel@joelfernandes.org> > > > Cc: Josh Triplett <josh@joshtriplett.org> > > > Cc: Uladzislau Rezki <urezki@gmail.com> > > > Cc: Steven Rostedt <rostedt@goodmis.org> > > > Cc: Lai Jiangshan <jiangshanlai@gmail.com> > > > Cc: Zqiang <qiang.zhang1211@gmail.com> > > > Cc: Ingo Molnar <mingo@redhat.com> > > > Cc: Waiman Long <longman@redhat.com> > > > Cc: Mark Rutland <mark.rutland@arm.com> > > > Cc: Thomas Gleixner <tglx@linutronix.de> > > > Cc: Vlastimil Babka <vbabka@suse.cz> > > > Cc: maged.michael@gmail.com > > > Cc: Mateusz Guzik <mjguzik@gmail.com> > > > Cc: Gary Guo <gary@garyguo.net> > > > Cc: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com> > > > Cc: rcu@vger.kernel.org > > > Cc: linux-mm@kvack.org > > > Cc: lkmm@lists.linux.dev > > > Cc: Nikita Popov <github@npopov.com> > > > 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. Much better! Acked-by: Paul E. McKenney <paulmck@kernel.org> > 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 >
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