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

Commit Message

Mathieu Desnoyers Sept. 28, 2024, 1:51 p.m. UTC

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 <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Sebastian Andrzej Siewior <[e-mail]>
Cc: "Paul E. McKenney" <[e-mail]>
Cc: Will Deacon <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Boqun Feng <[e-mail]>
Cc: Alan Stern <[e-mail]>
Cc: John Stultz <[e-mail]>
Cc: Neeraj Upadhyay <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Boqun Feng <[e-mail]>
Cc: Frederic Weisbecker <[e-mail]>
Cc: Joel Fernandes <[e-mail]>
Cc: Josh Triplett <[e-mail]>
Cc: Uladzislau Rezki <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Cc: Lai Jiangshan <[e-mail]>
Cc: Zqiang <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Waiman Long <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Cc: [e-mail]
Cc: Mateusz Guzik <[e-mail]>
Cc: Gary Guo <[e-mail]>
Cc: Jonas Oberhauser <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
---
 Documentation/RCU/rcu_dereference.rst | 34 +++++++++++++++++++++++----
 1 file changed, 29 insertions(+), 5 deletions(-)

Comments

Alan Stern Sept. 28, 2024, 2:58 p.m. UTC | #1

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 <[e-mail]>
> Cc: Greg Kroah-Hartman <[e-mail]>
> Cc: Sebastian Andrzej Siewior <[e-mail]>
> Cc: "Paul E. McKenney" <[e-mail]>
> Cc: Will Deacon <[e-mail]>
> Cc: Peter Zijlstra <[e-mail]>
> Cc: Boqun Feng <[e-mail]>
> Cc: Alan Stern <[e-mail]>
> Cc: John Stultz <[e-mail]>
> Cc: Neeraj Upadhyay <[e-mail]>
> Cc: Linus Torvalds <[e-mail]>
> Cc: Boqun Feng <[e-mail]>
> Cc: Frederic Weisbecker <[e-mail]>
> Cc: Joel Fernandes <[e-mail]>
> Cc: Josh Triplett <[e-mail]>
> Cc: Uladzislau Rezki <[e-mail]>
> Cc: Steven Rostedt <[e-mail]>
> Cc: Lai Jiangshan <[e-mail]>
> Cc: Zqiang <[e-mail]>
> Cc: Ingo Molnar <[e-mail]>
> Cc: Waiman Long <[e-mail]>
> Cc: Mark Rutland <[e-mail]>
> Cc: Thomas Gleixner <[e-mail]>
> Cc: Vlastimil Babka <[e-mail]>
> Cc: [e-mail]
> Cc: Mateusz Guzik <[e-mail]>
> Cc: Gary Guo <[e-mail]>
> Cc: Jonas Oberhauser <[e-mail]>
> Cc: [e-mail]
> Cc: [e-mail]
> Cc: [e-mail]
> ---
>  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
>

Mathieu Desnoyers Sept. 28, 2024, 3:09 p.m. UTC | #2

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
>>

Patch

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