[PATCH v1 0/2] Introduce ptr_eq() to preserve address dependency

[PATCH v1 0/2] Introduce ptr_eq() to preserve address dependency

[Mathieu Desnoyers]

Introduce ptr_eq() to compare two addresses while preserving the address
dependencies for later use of the address. It should be used when
comparing an address returned by rcu_dereference().

Thanks,

Mathieu

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: 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

Mathieu Desnoyers (2):
  compiler.h: Introduce ptr_eq() to preserve address dependency
  Documentation: RCU: Refer to ptr_eq()

 Documentation/RCU/rcu_dereference.rst | 32 +++++++++++---
 include/linux/compiler.h              | 63 +++++++++++++++++++++++++++
 2 files changed, 90 insertions(+), 5 deletions(-)

-- 
2.39.2

[PATCH v1 1/2] compiler.h: Introduce ptr_eq() to preserve address dependency

[Mathieu Desnoyers]

Compiler CSE and SSA GVN optimizations can cause the address dependency
of addresses returned by rcu_dereference to be lost when comparing those
pointers with either constants or previously loaded pointers.

Introduce ptr_eq() to compare two addresses while preserving the address
dependencies for later use of the address. It should be used when
comparing an address returned by rcu_dereference().

This is needed to prevent the compiler CSE and SSA GVN optimizations
from using @a (or @b) in places where the source refers to @b (or @a)
based on the fact that after the comparison, the two are known to be
equal, which does not preserve address dependencies and allows the
following misordering speculations:

- If @b is a constant, the compiler can issue the loads which depend
  on @a before loading @a.
- If @b is a register populated by a prior load, weakly-ordered
  CPUs can speculate loads which depend on @a before loading @a.

The same logic applies with @a and @b swapped.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Suggested-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Acked-by: "Paul E. McKenney" <paulmck@kernel.org>
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.
---
 include/linux/compiler.h | 63 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 63 insertions(+)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 2df665fa2964..75a378ae7af1 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -186,6 +186,69 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 	__asm__ ("" : "=r" (var) : "0" (var))
 #endif
 
+/*
+ * Compare two addresses while preserving the address dependencies for
+ * later use of the address. It should be used when comparing an address
+ * returned by rcu_dereference().
+ *
+ * This is needed to prevent the compiler CSE and SSA GVN optimizations
+ * from using @a (or @b) in places where the source refers to @b (or @a)
+ * based on the fact that after the comparison, the two are known to be
+ * equal, which does not preserve address dependencies and allows the
+ * following misordering speculations:
+ *
+ * - If @b is a constant, the compiler can issue the loads which depend
+ *   on @a before loading @a.
+ * - If @b is a register populated by a prior load, weakly-ordered
+ *   CPUs can speculate loads which depend on @a before loading @a.
+ *
+ * The same logic applies with @a and @b swapped.
+ *
+ * Return value: true if pointers are equal, false otherwise.
+ *
+ * The compiler barrier() is ineffective at fixing this issue. It does
+ * not prevent the compiler CSE from losing the address dependency:
+ *
+ * int fct_2_volatile_barriers(void)
+ * {
+ *     int *a, *b;
+ *
+ *     do {
+ *         a = READ_ONCE(p);
+ *         asm volatile ("" : : : "memory");
+ *         b = READ_ONCE(p);
+ *     } while (a != b);
+ *     asm volatile ("" : : : "memory");  <-- barrier()
+ *     return *b;
+ * }
+ *
+ * With gcc 14.2 (arm64):
+ *
+ * fct_2_volatile_barriers:
+ *         adrp    x0, .LANCHOR0
+ *         add     x0, x0, :lo12:.LANCHOR0
+ * .L2:
+ *         ldr     x1, [x0]  <-- x1 populated by first load.
+ *         ldr     x2, [x0]
+ *         cmp     x1, x2
+ *         bne     .L2
+ *         ldr     w0, [x1]  <-- x1 is used for access which should depend on b.
+ *         ret
+ *
+ * On weakly-ordered architectures, this lets CPU speculation use the
+ * result from the first load to speculate "ldr w0, [x1]" before
+ * "ldr x2, [x0]".
+ * Based on the RCU documentation, the control dependency does not
+ * prevent the CPU from speculating loads.
+ */
+static __always_inline
+int ptr_eq(const volatile void *a, const volatile void *b)
+{
+	OPTIMIZER_HIDE_VAR(a);
+	OPTIMIZER_HIDE_VAR(b);
+	return a == b;
+}
+
 #define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __COUNTER__)
 
 /**
-- 
2.39.2

[PATCH v1 2/2] Documentation: RCU: Refer to ptr_eq()

[Mathieu Desnoyers]

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
-- 
2.39.2

Re: [PATCH v1 2/2] Documentation: RCU: Refer to ptr_eq()

[Paul E. McKenney]

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.  :-/

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
> 

Re: [PATCH v1 2/2] Documentation: RCU: Refer to ptr_eq()

[Mathieu Desnoyers]

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

Re: [PATCH v1 2/2] Documentation: RCU: Refer to ptr_eq()

[Paul E. McKenney]

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
> 

Re: [PATCH v1 0/2] Introduce ptr_eq() to preserve address dependency

[Alan Stern]

On Sun, Sep 29, 2024 at 07:16:06AM -0400, Mathieu Desnoyers wrote:
> Introduce ptr_eq() to compare two addresses while preserving the address
> dependencies for later use of the address. It should be used when
> comparing an address returned by rcu_dereference().

Both patches are now much better.  Thank you.

Acked-by: Alan Stern <stern@rowland.harvard.edu>

Alan Stern