[PATCH 1/1] rcu: Check for have_rcu_nocb_mask instead of rcu_nocb_mask

[PATCH 1/1] rcu: Check for have_rcu_nocb_mask instead of rcu_nocb_mask

[Pranith Kumar]

Hi Paul,

This is something similar to what you found yesterday with tick_nohz_full mask.

--
Pranith

If we configure a kernel with CONFIG_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_NONE=y and
CONFIG_CPUMASK_OFFSTACK=n and do not pass in a rcu_nocb= boot parameter, the
cpumask rcu_nocb_mask can be garbage instead of NULL.

Hence this commit replaces checks for rcu_nocb_mask == NULL with a check for
have_rcu_nocb_mask.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
---
 kernel/rcu/tree_plugin.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index cedb020..15c00b5 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2542,7 +2542,7 @@ static void __init rcu_organize_nocb_kthreads(struct rcu_state *rsp)
 	struct rcu_data *rdp_leader = NULL;  /* Suppress misguided gcc warn. */
 	struct rcu_data *rdp_prev = NULL;
 
-	if (rcu_nocb_mask == NULL)
+	if (!have_rcu_nocb_mask)
 		return;
 #ifdef CONFIG_NO_HZ_FULL
 	cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
@@ -2575,7 +2575,7 @@ static void __init rcu_organize_nocb_kthreads(struct rcu_state *rsp)
 /* Prevent __call_rcu() from enqueuing callbacks on no-CBs CPUs */
 static bool init_nocb_callback_list(struct rcu_data *rdp)
 {
-	if (rcu_nocb_mask == NULL ||
+	if (have_rcu_nocb_mask == false ||
 	    !cpumask_test_cpu(rdp->cpu, rcu_nocb_mask))
 		return false;
 	rdp->nxttail[RCU_NEXT_TAIL] = NULL;
-- 
1.9.1

Re: [PATCH 1/1] rcu: Check for have_rcu_nocb_mask instead of rcu_nocb_mask

[Paul E. McKenney]

On Thu, Jul 17, 2014 at 05:30:11PM -0400, Pranith Kumar wrote:
> Hi Paul,
> 
> This is something similar to what you found yesterday with tick_nohz_full mask.
> 
> --
> Pranith
> 
> If we configure a kernel with CONFIG_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_NONE=y and
> CONFIG_CPUMASK_OFFSTACK=n and do not pass in a rcu_nocb= boot parameter, the
> cpumask rcu_nocb_mask can be garbage instead of NULL.
> 
> Hence this commit replaces checks for rcu_nocb_mask == NULL with a check for
> have_rcu_nocb_mask.
> 
> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>

Good catch!  Could you please forward-port this to current rcu/dev?
This now includes the fix you mentioned above, which generates conflicts.

Could you please also fix the use in rcu_is_nocb_cpu()?

And please see the comment below.

							Thanx, Paul

> ---
>  kernel/rcu/tree_plugin.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index cedb020..15c00b5 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -2542,7 +2542,7 @@ static void __init rcu_organize_nocb_kthreads(struct rcu_state *rsp)
>  	struct rcu_data *rdp_leader = NULL;  /* Suppress misguided gcc warn. */
>  	struct rcu_data *rdp_prev = NULL;
> 
> -	if (rcu_nocb_mask == NULL)
> +	if (!have_rcu_nocb_mask)
>  		return;
>  #ifdef CONFIG_NO_HZ_FULL
>  	cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
> @@ -2575,7 +2575,7 @@ static void __init rcu_organize_nocb_kthreads(struct rcu_state *rsp)
>  /* Prevent __call_rcu() from enqueuing callbacks on no-CBs CPUs */
>  static bool init_nocb_callback_list(struct rcu_data *rdp)
>  {
> -	if (rcu_nocb_mask == NULL ||
> +	if (have_rcu_nocb_mask == false ||

This should really be "if (!have_rcu_nocb_mask".

Of course, the original should have been "if (!rcu_nocb_mask", but old
habits die hard.  ;-)

>  	    !cpumask_test_cpu(rdp->cpu, rcu_nocb_mask))
>  		return false;
>  	rdp->nxttail[RCU_NEXT_TAIL] = NULL;
> -- 
> 1.9.1
> 

Re: [PATCH 1/1] rcu: Check for have_rcu_nocb_mask instead of rcu_nocb_mask

[Pranith Kumar]

On 07/17/2014 07:49 PM, Paul E. McKenney wrote:
> On Thu, Jul 17, 2014 at 05:30:11PM -0400, Pranith Kumar wrote:
>> Hi Paul,
>>
>> This is something similar to what you found yesterday with tick_nohz_full mask.
>>
>> --
>> Pranith
>>
>> If we configure a kernel with CONFIG_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_NONE=y and
>> CONFIG_CPUMASK_OFFSTACK=n and do not pass in a rcu_nocb= boot parameter, the
>> cpumask rcu_nocb_mask can be garbage instead of NULL.
>>
>> Hence this commit replaces checks for rcu_nocb_mask == NULL with a check for
>> have_rcu_nocb_mask.
>>
>> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
> 
> Good catch!  Could you please forward-port this to current rcu/dev?
> This now includes the fix you mentioned above, which generates conflicts.
> 
> Could you please also fix the use in rcu_is_nocb_cpu()?
> 
> And please see the comment below.
> 

Please find the fixed patch.

--
Pranith


From: Pranith Kumar <bobby.prani@gmail.com>
Date: Thu, 17 Jul 2014 20:02:54 -0400
Subject: [PATCH 1/1] rcu: Feedback comments

If we configure a kernel with CONFIG_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_NONE=y and
CONFIG_CPUMASK_OFFSTACK=n and do not pass in a rcu_nocb= boot parameter, the
cpumask rcu_nocb_mask can be garbage instead of NULL.

Hence this commit replaces checks for rcu_nocb_mask == NULL with a check for
have_rcu_nocb_mask.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
---
 kernel/rcu/tree_plugin.h | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index ce8c331..8f987c1 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2053,9 +2053,10 @@ static void rcu_init_one_nocb(struct rcu_node *rnp)
 /* Is the specified CPU a no-CBs CPU? */
 bool rcu_is_nocb_cpu(int cpu)
 {
-	if (have_rcu_nocb_mask)
-		return cpumask_test_cpu(cpu, rcu_nocb_mask);
-	return false;
+	if (!have_rcu_nocb_mask)
+		return false;
+
+	return cpumask_test_cpu(cpu, rcu_nocb_mask);
 }
 #endif /* #ifndef CONFIG_RCU_NOCB_CPU_ALL */
 
@@ -2540,7 +2541,7 @@ static void __init rcu_organize_nocb_kthreads(struct rcu_state *rsp)
 	struct rcu_data *rdp_leader = NULL;  /* Suppress misguided gcc warn. */
 	struct rcu_data *rdp_prev = NULL;
 
-	if (rcu_nocb_mask == NULL)
+	if (!have_rcu_nocb_mask)
 		return;
 #if defined(CONFIG_NO_HZ_FULL) && !defined(CONFIG_NO_HZ_FULL_ALL)
 	if (tick_nohz_full_running)
@@ -2574,9 +2575,9 @@ static void __init rcu_organize_nocb_kthreads(struct rcu_state *rsp)
 /* Prevent __call_rcu() from enqueuing callbacks on no-CBs CPUs */
 static bool init_nocb_callback_list(struct rcu_data *rdp)
 {
-	if (rcu_nocb_mask == NULL ||
-	    !cpumask_test_cpu(rdp->cpu, rcu_nocb_mask))
+	if (!rcu_is_nocb_cpu(rdp->cpu))
 		return false;
+
 	rdp->nxttail[RCU_NEXT_TAIL] = NULL;
 	return true;
 }
-- 
1.9.1

Re: [PATCH 1/1] rcu: Check for have_rcu_nocb_mask instead of rcu_nocb_mask

[Paul E. McKenney]

On Thu, Jul 17, 2014 at 08:11:01PM -0400, Pranith Kumar wrote:
> On 07/17/2014 07:49 PM, Paul E. McKenney wrote:
> > On Thu, Jul 17, 2014 at 05:30:11PM -0400, Pranith Kumar wrote:
> >> Hi Paul,
> >>
> >> This is something similar to what you found yesterday with tick_nohz_full mask.
> >>
> >> --
> >> Pranith
> >>
> >> If we configure a kernel with CONFIG_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_NONE=y and
> >> CONFIG_CPUMASK_OFFSTACK=n and do not pass in a rcu_nocb= boot parameter, the
> >> cpumask rcu_nocb_mask can be garbage instead of NULL.
> >>
> >> Hence this commit replaces checks for rcu_nocb_mask == NULL with a check for
> >> have_rcu_nocb_mask.
> >>
> >> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
> > 
> > Good catch!  Could you please forward-port this to current rcu/dev?
> > This now includes the fix you mentioned above, which generates conflicts.
> > 
> > Could you please also fix the use in rcu_is_nocb_cpu()?
> > 
> > And please see the comment below.
> > 
> 
> Please find the fixed patch.
> 
> --
> Pranith
> 
> 
> From: Pranith Kumar <bobby.prani@gmail.com>
> Date: Thu, 17 Jul 2014 20:02:54 -0400
> Subject: [PATCH 1/1] rcu: Feedback comments
> 
> If we configure a kernel with CONFIG_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_NONE=y and
> CONFIG_CPUMASK_OFFSTACK=n and do not pass in a rcu_nocb= boot parameter, the
> cpumask rcu_nocb_mask can be garbage instead of NULL.
> 
> Hence this commit replaces checks for rcu_nocb_mask == NULL with a check for
> have_rcu_nocb_mask.
> 
> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
> ---
>  kernel/rcu/tree_plugin.h | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index ce8c331..8f987c1 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -2053,9 +2053,10 @@ static void rcu_init_one_nocb(struct rcu_node *rnp)
>  /* Is the specified CPU a no-CBs CPU? */
>  bool rcu_is_nocb_cpu(int cpu)
>  {
> -	if (have_rcu_nocb_mask)
> -		return cpumask_test_cpu(cpu, rcu_nocb_mask);
> -	return false;
> +	if (!have_rcu_nocb_mask)
> +		return false;
> +
> +	return cpumask_test_cpu(cpu, rcu_nocb_mask);

Hmmm...  That was a subtle way to tell me that my feedback was bogus.  ;-)
I guess I need to stop reviewing patches at 2AM local time.

Anyway, you are correct, this was OK to start with, so I am dropping
this hunk.

I queued the other two for 3.18.

							Thanx, Paul

>  }
>  #endif /* #ifndef CONFIG_RCU_NOCB_CPU_ALL */
> 
> @@ -2540,7 +2541,7 @@ static void __init rcu_organize_nocb_kthreads(struct rcu_state *rsp)
>  	struct rcu_data *rdp_leader = NULL;  /* Suppress misguided gcc warn. */
>  	struct rcu_data *rdp_prev = NULL;
> 
> -	if (rcu_nocb_mask == NULL)
> +	if (!have_rcu_nocb_mask)
>  		return;
>  #if defined(CONFIG_NO_HZ_FULL) && !defined(CONFIG_NO_HZ_FULL_ALL)
>  	if (tick_nohz_full_running)
> @@ -2574,9 +2575,9 @@ static void __init rcu_organize_nocb_kthreads(struct rcu_state *rsp)
>  /* Prevent __call_rcu() from enqueuing callbacks on no-CBs CPUs */
>  static bool init_nocb_callback_list(struct rcu_data *rdp)
>  {
> -	if (rcu_nocb_mask == NULL ||
> -	    !cpumask_test_cpu(rdp->cpu, rcu_nocb_mask))
> +	if (!rcu_is_nocb_cpu(rdp->cpu))
>  		return false;
> +
>  	rdp->nxttail[RCU_NEXT_TAIL] = NULL;
>  	return true;
>  }
> -- 
> 1.9.1
> 
> 
> --
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Re: [PATCH 1/1] rcu: Check for have_rcu_nocb_mask instead of rcu_nocb_mask

[Pranith Kumar]

On 07/19/2014 02:10 AM, Paul E. McKenney wrote:

>> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
>> index ce8c331..8f987c1 100644
>> --- a/kernel/rcu/tree_plugin.h
>> +++ b/kernel/rcu/tree_plugin.h
>> @@ -2053,9 +2053,10 @@ static void rcu_init_one_nocb(struct rcu_node *rnp)
>>  /* Is the specified CPU a no-CBs CPU? */
>>  bool rcu_is_nocb_cpu(int cpu)
>>  {
>> -	if (have_rcu_nocb_mask)
>> -		return cpumask_test_cpu(cpu, rcu_nocb_mask);
>> -	return false;
>> +	if (!have_rcu_nocb_mask)
>> +		return false;
>> +
>> +	return cpumask_test_cpu(cpu, rcu_nocb_mask);
> 
> Hmmm...  That was a subtle way to tell me that my feedback was bogus.  ;-)
> I guess I need to stop reviewing patches at 2AM local time.
> 
> Anyway, you are correct, this was OK to start with, so I am dropping
> this hunk.
> 

I thought you wanted me to change the check of the have_rcu_nocb_mask, and I
really think the new version is better. But yes, more of a style change :)

--
Pranith