Re: [PATCH] fib_trie: rcu_assign_pointer warning fix

Re: [PATCH] fib_trie: rcu_assign_pointer warning fix

[David Miller]

From: Stephen Hemminger <shemminger@vyatta.com>
Date: Mon, 11 Feb 2008 16:59:54 -0800

linux-kernel added to CC:, any change to generic kernel infrastructure
should be posted there

> Eliminate warnings when rcu_assign_pointer is used with unsigned long.
> It is reasonable to use RCU with non-pointer values so allow it for general
> use.  Add a comment to explain the if test.
> 
> Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
> ---
>  include/linux/rcupdate.h |   13 +++++++------
>  1 files changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 37a642c..c44ac87 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -172,14 +172,15 @@ struct rcu_head {
>   * structure after the pointer assignment.  More importantly, this
>   * call documents which pointers will be dereferenced by RCU read-side
>   * code.
> + *
> + * If value is the NULL (constant 0), then no barrier is needed.
>   */
>  
> -#define rcu_assign_pointer(p, v) \
> -	({ \
> -		if (!__builtin_constant_p(v) || \
> -		    ((v) != NULL)) \
> -			smp_wmb(); \
> -		(p) = (v); \
> +#define rcu_assign_pointer(p, v)			\
> +	({						\
> +		if (!(__builtin_constant_p(v) && v))	\
> +			smp_wmb();			\
> +		(p) = (v);				\
>  	})
>  
>  /**
> -- 
> 1.5.3.8
> 

Re: [PATCH] fib_trie: rcu_assign_pointer warning fix

[Jarek Poplawski]

On 12-02-2008 02:16, David Miller wrote:
> From: Stephen Hemminger <shemminger@vyatta.com>
> Date: Mon, 11 Feb 2008 16:59:54 -0800
> 
> linux-kernel added to CC:, any change to generic kernel infrastructure
> should be posted there
> 
>> Eliminate warnings when rcu_assign_pointer is used with unsigned long.
>> It is reasonable to use RCU with non-pointer values so allow it for general
>> use.  Add a comment to explain the if test.
>>
>> Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
>> ---
>>  include/linux/rcupdate.h |   13 +++++++------
>>  1 files changed, 7 insertions(+), 6 deletions(-)
>>
>> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
>> index 37a642c..c44ac87 100644
>> --- a/include/linux/rcupdate.h
>> +++ b/include/linux/rcupdate.h
>> @@ -172,14 +172,15 @@ struct rcu_head {
>>   * structure after the pointer assignment.  More importantly, this
>>   * call documents which pointers will be dereferenced by RCU read-side
>>   * code.
>> + *
>> + * If value is the NULL (constant 0), then no barrier is needed.
>>   */
>>  
>> -#define rcu_assign_pointer(p, v) \
>> -	({ \
>> -		if (!__builtin_constant_p(v) || \
>> -		    ((v) != NULL)) \
>> -			smp_wmb(); \
>> -		(p) = (v); \
>> +#define rcu_assign_pointer(p, v)			\
>> +	({						\
>> +		if (!(__builtin_constant_p(v) && v))	\

...But, "If value is the NULL (constant 0)" we have:

if (!(1 && NULL != 0)) ==> if (!(0)) and the barrier is used?!

>> +			smp_wmb();			\
>> +		(p) = (v);				\
>>  	})
>>  
>>  /**


Regards,
Jarek P.

Re: [PATCH] fib_trie: rcu_assign_pointer warning fix

[Paul E. McKenney]

On Tue, Feb 12, 2008 at 08:57:14AM +0000, Jarek Poplawski wrote:
> On 12-02-2008 02:16, David Miller wrote:
> > From: Stephen Hemminger <shemminger@vyatta.com>
> > Date: Mon, 11 Feb 2008 16:59:54 -0800
> > 
> > linux-kernel added to CC:, any change to generic kernel infrastructure
> > should be posted there
> > 
> >> Eliminate warnings when rcu_assign_pointer is used with unsigned long.
> >> It is reasonable to use RCU with non-pointer values so allow it for general
> >> use.  Add a comment to explain the if test.
> >>
> >> Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
> >> ---
> >>  include/linux/rcupdate.h |   13 +++++++------
> >>  1 files changed, 7 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> >> index 37a642c..c44ac87 100644
> >> --- a/include/linux/rcupdate.h
> >> +++ b/include/linux/rcupdate.h
> >> @@ -172,14 +172,15 @@ struct rcu_head {
> >>   * structure after the pointer assignment.  More importantly, this
> >>   * call documents which pointers will be dereferenced by RCU read-side
> >>   * code.
> >> + *
> >> + * If value is the NULL (constant 0), then no barrier is needed.
> >>   */
> >>  
> >> -#define rcu_assign_pointer(p, v) \
> >> -	({ \
> >> -		if (!__builtin_constant_p(v) || \
> >> -		    ((v) != NULL)) \
> >> -			smp_wmb(); \
> >> -		(p) = (v); \
> >> +#define rcu_assign_pointer(p, v)			\
> >> +	({						\
> >> +		if (!(__builtin_constant_p(v) && v))	\
> 
> ...But, "If value is the NULL (constant 0)" we have:
> 
> if (!(1 && NULL != 0)) ==> if (!(0)) and the barrier is used?!

"All programmers are blind, especially me."

You are right, Jarek.  I ran this through gcc, and the following
comes close:

#define rcu_assign_pointer(p, v) \
	({ \
		if (!__builtin_constant_p(v) || (v)) \
			smp_wmb(); \
		(p) = (v); \
	})

But I am concerned about the following case:

	rcu_assign_pointer(global_index, 0);

	. . .

	x = global_array[rcu_dereference(global_index)];

Since arrays have a zero-th element, we would really want a memory
barrier in this case.

So how about leaving the index-unfriendly version of rcu_assign_pointer()
and adding an rcu_assign_index() as follows?

							Thanx, Paul

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---

 rcupdate.h |   18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff -urpNa -X dontdiff linux-2.6.24/include/linux/rcupdate.h linux-2.6.24-rai/include/linux/rcupdate.h
--- linux-2.6.24/include/linux/rcupdate.h	2008-01-24 14:58:37.000000000 -0800
+++ linux-2.6.24-rai/include/linux/rcupdate.h	2008-02-12 08:04:59.000000000 -0800
@@ -278,6 +278,24 @@ extern struct lockdep_map rcu_lock_map;
 					})
 
 /**
+ * rcu_assign_index - assign (publicize) a index of a newly
+ * initialized array elementg that will be dereferenced by RCU
+ * read-side critical sections.  Returns the value assigned.
+ *
+ * Inserts memory barriers on architectures that require them
+ * (pretty much all of them other than x86), and also prevents
+ * the compiler from reordering the code that initializes the
+ * structure after the index assignment.  More importantly, this
+ * call documents which indexes will be dereferenced by RCU read-side
+ * code.
+ */
+
+#define rcu_assign_index(p, v)	({ \
+					smp_wmb(); \
+					(p) = (v); \
+				})
+
+/**
  * synchronize_sched - block until all CPUs have exited any non-preemptive
  * kernel code sequences.
  *

Re: [PATCH] fib_trie: rcu_assign_pointer warning fix

[Jarek Poplawski]

On Tue, Feb 12, 2008 at 08:07:29AM -0800, Paul E. McKenney wrote:
...
> "All programmers are blind, especially me."

Hmm... I got it my way: you - superheroes - sometimes seem to be just
like us - common people... (Probably early in the morning, before
dressing your funny costumes?) 

> You are right, Jarek.  I ran this through gcc, and the following
> comes close:
> 
> #define rcu_assign_pointer(p, v) \
> 	({ \
> 		if (!__builtin_constant_p(v) || (v)) \
> 			smp_wmb(); \
> 		(p) = (v); \
> 	})
> 
> But I am concerned about the following case:
> 
> 	rcu_assign_pointer(global_index, 0);
> 
> 	. . .
> 
> 	x = global_array[rcu_dereference(global_index)];
> 
> Since arrays have a zero-th element, we would really want a memory
> barrier in this case.

It seems the above version of this macro uses the barrier for 0, but
if I miss something, or for these other: documenting reasons, then of
course you are right.

> 
> So how about leaving the index-unfriendly version of rcu_assign_pointer()
> and adding an rcu_assign_index() as follows?
> 
> 							Thanx, Paul
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
> 
>  rcupdate.h |   18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff -urpNa -X dontdiff linux-2.6.24/include/linux/rcupdate.h linux-2.6.24-rai/include/linux/rcupdate.h
> --- linux-2.6.24/include/linux/rcupdate.h	2008-01-24 14:58:37.000000000 -0800
> +++ linux-2.6.24-rai/include/linux/rcupdate.h	2008-02-12 08:04:59.000000000 -0800
> @@ -278,6 +278,24 @@ extern struct lockdep_map rcu_lock_map;
>  					})
>  
>  /**
> + * rcu_assign_index - assign (publicize) a index of a newly
> + * initialized array elementg that will be dereferenced by RCU

---------------------------^
 + * initialized array element that will be dereferenced by RCU

Regards,
Jarek P.

Re: [PATCH] fib_trie: rcu_assign_pointer warning fix

[Jarek Poplawski]

On Tue, Feb 12, 2008 at 08:32:18PM +0100, Jarek Poplawski wrote:
...
> It seems the above version of this macro uses the barrier for 0, but
> if I miss something, or for these other: documenting reasons,

...or __builtin_constants could be used for indexing (?!),

> then of
> course you are right.

Jarek P.

Re: [PATCH] fib_trie: rcu_assign_pointer warning fix

[Paul E. McKenney]

On Tue, Feb 12, 2008 at 08:46:30PM +0100, Jarek Poplawski wrote:
> On Tue, Feb 12, 2008 at 08:32:18PM +0100, Jarek Poplawski wrote:
> ...
> > It seems the above version of this macro uses the barrier for 0, but
> > if I miss something, or for these other: documenting reasons,
> 
> ...or __builtin_constants could be used for indexing (?!),

Yep.  For example:

	elem[0].next = 1;
	rcu_assign_index(global_index, 0);

							Thanx, Paul

> > then of
> > course you are right.
> 
> Jarek P.

Re: [PATCH] fib_trie: rcu_assign_pointer warning fix

[David Miller]

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Date: Mon, 11 Feb 2008 17:27:41 -0800

> On Mon, Feb 11, 2008 at 04:59:54PM -0800, Stephen Hemminger wrote:
> > Eliminate warnings when rcu_assign_pointer is used with unsigned long.
> > It is reasonable to use RCU with non-pointer values so allow it for general
> > use.  Add a comment to explain the if test.
> 
> Good catch!!!  (An apologies for the hassle!!!)
> 
> Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Thanks for reviewing Paul, I'll apply this.

[PATCH] fib_trie: rcu_assign_pointer warning fix

[Stephen Hemminger]

Eliminate warnings when rcu_assign_pointer is used with unsigned long.
It is reasonable to use RCU with non-pointer values so allow it for general
use.  Add a comment to explain the if test.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
---
 include/linux/rcupdate.h |   13 +++++++------
 1 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 37a642c..c44ac87 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -172,14 +172,15 @@ struct rcu_head {
  * structure after the pointer assignment.  More importantly, this
  * call documents which pointers will be dereferenced by RCU read-side
  * code.
+ *
+ * If value is the NULL (constant 0), then no barrier is needed.
  */
 
-#define rcu_assign_pointer(p, v) \
-	({ \
-		if (!__builtin_constant_p(v) || \
-		    ((v) != NULL)) \
-			smp_wmb(); \
-		(p) = (v); \
+#define rcu_assign_pointer(p, v)			\
+	({						\
+		if (!(__builtin_constant_p(v) && v))	\
+			smp_wmb();			\
+		(p) = (v);				\
 	})
 
 /**
-- 
1.5.3.8