[Linux-kernel-mentees] [PATCH] exit.c: Fix Sparse errors and warnings

[Linux-kernel-mentees] [PATCH] exit.c: Fix Sparse errors and warnings

[madhuparnabhowmik10]

From: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>

This patch fixes the following sparse error:
kernel/exit.c:627:25: error: incompatible types in comparison expression
caused due to accessing rcu protected pointer without using rcu primitives.

And the following warning:

kernel/exit.c:626:40: warning: incorrect type in assignment

Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
---
 kernel/exit.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index bcbd59888e67..c5a9d6360440 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -623,8 +623,8 @@ static void forget_original_parent(struct task_struct *father,
 	reaper = find_new_reaper(father, reaper);
 	list_for_each_entry(p, &father->children, sibling) {
 		for_each_thread(p, t) {
-			t->real_parent = reaper;
-			BUG_ON((!t->ptrace) != (t->parent == father));
+			rcu_assign_pointer(t->real_parent, reaper);
+			BUG_ON((!t->ptrace) != (rcu_access_pointer(t->parent) == father));
 			if (likely(!t->ptrace))
 				t->parent = t->real_parent;
 			if (t->pdeath_signal)
-- 
2.17.1

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Re: [Linux-kernel-mentees] [PATCH] exit.c: Fix Sparse errors and warnings

[Christian Brauner]

On Tue, Jan 28, 2020 at 10:50:08PM +0530, madhuparnabhowmik10@gmail.com wrote:
> From: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
> 
> This patch fixes the following sparse error:
> kernel/exit.c:627:25: error: incompatible types in comparison expression
> caused due to accessing rcu protected pointer without using rcu primitives.
> 
> And the following warning:
> 
> kernel/exit.c:626:40: warning: incorrect type in assignment
> 
> Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>

Thanks, looks good to me!
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
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Re: [Linux-kernel-mentees] [PATCH] exit.c: Fix Sparse errors and warnings

[Oleg Nesterov]

On 01/28, madhuparnabhowmik10@gmail.com wrote:
>
> kernel/exit.c:626:40: warning: incorrect type in assignment
> 
> Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
> ---
>  kernel/exit.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/exit.c b/kernel/exit.c
> index bcbd59888e67..c5a9d6360440 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -623,8 +623,8 @@ static void forget_original_parent(struct task_struct *father,
>  	reaper = find_new_reaper(father, reaper);
>  	list_for_each_entry(p, &father->children, sibling) {
>  		for_each_thread(p, t) {
> -			t->real_parent = reaper;
> -			BUG_ON((!t->ptrace) != (t->parent == father));
> +			rcu_assign_pointer(t->real_parent, reaper);

Another case when RCU_INIT_POINTER() makes more sense (although to me it
too looks confusing). We didn't modify the new parent.

Oleg.

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Re: [Linux-kernel-mentees] [PATCH] exit.c: Fix Sparse errors and warnings

[Madhuparna Bhowmik]

On Wed, Jan 29, 2020 at 01:30:47PM +0100, Oleg Nesterov wrote:
> On 01/28, madhuparnabhowmik10@gmail.com wrote:
> >
> > kernel/exit.c:626:40: warning: incorrect type in assignment
> > 
> > Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
> > ---
> >  kernel/exit.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/exit.c b/kernel/exit.c
> > index bcbd59888e67..c5a9d6360440 100644
> > --- a/kernel/exit.c
> > +++ b/kernel/exit.c
> > @@ -623,8 +623,8 @@ static void forget_original_parent(struct task_struct *father,
> >  	reaper = find_new_reaper(father, reaper);
> >  	list_for_each_entry(p, &father->children, sibling) {
> >  		for_each_thread(p, t) {
> > -			t->real_parent = reaper;
> > -			BUG_ON((!t->ptrace) != (t->parent == father));
> > +			rcu_assign_pointer(t->real_parent, reaper);
> 
> Another case when RCU_INIT_POINTER() makes more sense (although to me it
> too looks confusing). We didn't modify the new parent.
>
Alright, I will resend this patch with RCU_INIT_POINTER().

Thank you,
Madhuparna

> Oleg.
> 
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Re: [Linux-kernel-mentees] [PATCH] exit.c: Fix Sparse errors and warnings

[Christian Brauner]

On Thu, Jan 30, 2020 at 12:45:26PM +0100, Christian Brauner wrote:
> On January 30, 2020 12:33:41 PM GMT+01:00, Oleg Nesterov <oleg@redhat.com> wrote:
> >On 01/30, Christian Brauner wrote:
> >>
> >> On Thu, Jan 30, 2020 at 11:50:28AM +0530,
> >madhuparnabhowmik10@gmail.com wrote:
> >> > From: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
> >> >
> >> > This patch fixes the following sparse error:
> >> > kernel/exit.c:627:25: error: incompatible types in comparison
> >expression
> >> >
> >> > And the following warning:
> >> > kernel/exit.c:626:40: warning: incorrect type in assignment
> >> >
> >> > Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
> >>
> >> I think the previous version was already fine but hopefully
> >> RCU_INIT_POINTER() really saves some overhead. In any case:
> >
> >It is not about overhead, RCU_INIT_POINTER() documents the fact that we
> >didn't make any changes to the new parent, we only need to change the
> >pointer.
> 
> Right, I wasn't complaining.  RCU_INIT_POINTER() claims that it has less overhead than rcu_assign_pointer().
> So that is an additional argument for it.
> 
> >
> >And btw, I don't really understand the __rcu annotations. Say,
> >according
> >to sparse this code is wrong:
> >
> >	int __rcu *P;
> >
> >	void func(int *p)
> >	{
> >		P = p;
> >	}
> >
> >OK, although quite possibly it is fine.
> >
> >However, this code
> >
> >	int __rcu *P;
> >
> >	void func(int __rcu *p)
> >	{
> >		*p = 10;
> >	       	P = p;
> >	}
> >
> >is almost certainly wrong but sparse is happy, asn is the same.
> 
> That's more an argument to fix sparse I guess?
> The annotations themselves are rather useful I think.
> They at least help me when reading the code.
> It's not that rcu lifetimes are trivial and anything that helps remind me that an object wants rcu semantics I'm happy to take it. :)
> 
> >
> >
> >> Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
> >
> >Acked-by: Oleg Nesterov <oleg@redhat.com>

Thanks, applied for post -rc1.
Christian
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Re: [Linux-kernel-mentees] [PATCH] exit.c: Fix Sparse errors and warnings

[Christian Brauner]

On January 30, 2020 12:33:41 PM GMT+01:00, Oleg Nesterov <oleg@redhat.com> wrote:
>On 01/30, Christian Brauner wrote:
>>
>> On Thu, Jan 30, 2020 at 11:50:28AM +0530,
>madhuparnabhowmik10@gmail.com wrote:
>> > From: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
>> >
>> > This patch fixes the following sparse error:
>> > kernel/exit.c:627:25: error: incompatible types in comparison
>expression
>> >
>> > And the following warning:
>> > kernel/exit.c:626:40: warning: incorrect type in assignment
>> >
>> > Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
>>
>> I think the previous version was already fine but hopefully
>> RCU_INIT_POINTER() really saves some overhead. In any case:
>
>It is not about overhead, RCU_INIT_POINTER() documents the fact that we
>didn't make any changes to the new parent, we only need to change the
>pointer.

Right, I wasn't complaining.  RCU_INIT_POINTER() claims that it has less overhead than rcu_assign_pointer().
So that is an additional argument for it.

>
>And btw, I don't really understand the __rcu annotations. Say,
>according
>to sparse this code is wrong:
>
>	int __rcu *P;
>
>	void func(int *p)
>	{
>		P = p;
>	}
>
>OK, although quite possibly it is fine.
>
>However, this code
>
>	int __rcu *P;
>
>	void func(int __rcu *p)
>	{
>		*p = 10;
>	       	P = p;
>	}
>
>is almost certainly wrong but sparse is happy, asn is the same.

That's more an argument to fix sparse I guess?
The annotations themselves are rather useful I think.
They at least help me when reading the code.
It's not that rcu lifetimes are trivial and anything that helps remind me that an object wants rcu semantics I'm happy to take it. :)

>
>
>> Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
>
>Acked-by: Oleg Nesterov <oleg@redhat.com>

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Re: [Linux-kernel-mentees] [PATCH] exit.c: Fix Sparse errors and warnings

[Oleg Nesterov]

On 01/30, Christian Brauner wrote:
>
> On Thu, Jan 30, 2020 at 11:50:28AM +0530, madhuparnabhowmik10@gmail.com wrote:
> > From: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
> >
> > This patch fixes the following sparse error:
> > kernel/exit.c:627:25: error: incompatible types in comparison expression
> >
> > And the following warning:
> > kernel/exit.c:626:40: warning: incorrect type in assignment
> >
> > Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
>
> I think the previous version was already fine but hopefully
> RCU_INIT_POINTER() really saves some overhead. In any case:

It is not about overhead, RCU_INIT_POINTER() documents the fact that we
didn't make any changes to the new parent, we only need to change the
pointer.

And btw, I don't really understand the __rcu annotations. Say, according
to sparse this code is wrong:

	int __rcu *P;

	void func(int *p)
	{
		P = p;
	}

OK, although quite possibly it is fine.

However, this code

	int __rcu *P;

	void func(int __rcu *p)
	{
		*p = 10;
	       	P = p;
	}

is almost certainly wrong but sparse is happy, asn is the same.


> Acked-by: Christian Brauner <christian.brauner@ubuntu.com>

Acked-by: Oleg Nesterov <oleg@redhat.com>

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Re: [Linux-kernel-mentees] [PATCH] exit.c: Fix Sparse errors and warnings

[Christian Brauner]

On Thu, Jan 30, 2020 at 11:50:28AM +0530, madhuparnabhowmik10@gmail.com wrote:
> From: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
> 
> This patch fixes the following sparse error:
> kernel/exit.c:627:25: error: incompatible types in comparison expression
> 
> And the following warning:
> kernel/exit.c:626:40: warning: incorrect type in assignment
> 
> Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>

I think the previous version was already fine but hopefully
RCU_INIT_POINTER() really saves some overhead. In any case:

Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
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[Linux-kernel-mentees] [PATCH] exit.c: Fix Sparse errors and warnings

[madhuparnabhowmik10]

From: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>

This patch fixes the following sparse error:
kernel/exit.c:627:25: error: incompatible types in comparison expression

And the following warning:
kernel/exit.c:626:40: warning: incorrect type in assignment

Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
---
 kernel/exit.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index bcbd59888e67..daf827a4aa25 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -623,8 +623,8 @@ static void forget_original_parent(struct task_struct *father,
 	reaper = find_new_reaper(father, reaper);
 	list_for_each_entry(p, &father->children, sibling) {
 		for_each_thread(p, t) {
-			t->real_parent = reaper;
-			BUG_ON((!t->ptrace) != (t->parent == father));
+			RCU_INIT_POINTER(t->real_parent, reaper);
+			BUG_ON((!t->ptrace) != (rcu_access_pointer(t->parent) == father));
 			if (likely(!t->ptrace))
 				t->parent = t->real_parent;
 			if (t->pdeath_signal)
-- 
2.17.1

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