[PATCH] mm: Use BUG_ON instead of if condition followed by BUG

[PATCH] mm: Use BUG_ON instead of if condition followed by BUG

[cgel.zte]

From: chiminghao <chi.minghao@zte.com.cn>

Fix the following coccinelle report:
./mm/memory_hotplug.c:2210:2-5:
WARNING  Use BUG_ON instead of if condition followed by BUG.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: chiminghao <chi.minghao@zte.com.cn>
---
 mm/memory_hotplug.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 3de7933e5302..aecb12bb7513 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -2212,8 +2212,7 @@ void __remove_memory(u64 start, u64 size)
 	 * trigger BUG() if some memory is not offlined prior to calling this
 	 * function
 	 */
-	if (try_remove_memory(start, size))
-		BUG();
+	BUG_ON(try_remove_memory(start, size));
 }
 
 /*
-- 
2.25.1

Re: [PATCH] mm: Use BUG_ON instead of if condition followed by BUG

[David Hildenbrand]

On 24.11.21 04:08, cgel.zte@gmail.com wrote:
> From: chiminghao <chi.minghao@zte.com.cn>

"mm/memory_hotplug: Use BUG_ON instead of if condition followed by BUG"

Would be better

> 
> Fix the following coccinelle report:
> ./mm/memory_hotplug.c:2210:2-5:
> WARNING  Use BUG_ON instead of if condition followed by BUG.
> 
> Reported-by: Zeal Robot <zealci@zte.com.cn>
> Signed-off-by: chiminghao <chi.minghao@zte.com.cn>
> ---
>  mm/memory_hotplug.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 3de7933e5302..aecb12bb7513 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -2212,8 +2212,7 @@ void __remove_memory(u64 start, u64 size)
>  	 * trigger BUG() if some memory is not offlined prior to calling this
>  	 * function
>  	 */
> -	if (try_remove_memory(start, size))
> -		BUG();
> +	BUG_ON(try_remove_memory(start, size));
>  }
>  
>  /*
> 

Acked-by: David Hildenbrand <david@redhat.com>


-- 
Thanks,

David / dhildenb

Re: [PATCH] mm: Use BUG_ON instead of if condition followed by BUG

[Matthew Wilcox]

On Wed, Nov 24, 2021 at 03:08:49AM +0000, cgel.zte@gmail.com wrote:
> From: chiminghao <chi.minghao@zte.com.cn>
> 
> Fix the following coccinelle report:
> ./mm/memory_hotplug.c:2210:2-5:
> WARNING  Use BUG_ON instead of if condition followed by BUG.

What coccinelle script is reporting this?

> -	if (try_remove_memory(start, size))
> -		BUG();
> +	BUG_ON(try_remove_memory(start, size));

I really, really, really do not like this.  For functions with
side-effects, this is bad style.  If it's a pure predicate, then
sure, but this is bad.

Re: [PATCH] mm: Use BUG_ON instead of if condition followed by BUG

[Andrew Morton]

On Wed, 24 Nov 2021 13:23:42 +0000 Matthew Wilcox <willy@infradead.org> wrote:

> On Wed, Nov 24, 2021 at 03:08:49AM +0000, cgel.zte@gmail.com wrote:
> > From: chiminghao <chi.minghao@zte.com.cn>
> > 
> > Fix the following coccinelle report:
> > ./mm/memory_hotplug.c:2210:2-5:
> > WARNING  Use BUG_ON instead of if condition followed by BUG.
> 
> What coccinelle script is reporting this?
> 
> > -	if (try_remove_memory(start, size))
> > -		BUG();
> > +	BUG_ON(try_remove_memory(start, size));
> 
> I really, really, really do not like this.  For functions with
> side-effects, this is bad style.  If it's a pure predicate, then
> sure, but this is bad.

I don't like it either.  Yes, BUG() is special but it's such dangerous
practice.  I'd vote to change coccinelle.

Re: [PATCH] mm: Use BUG_ON instead of if condition followed by BUG

[John Hubbard]

On 11/24/21 14:27, Andrew Morton wrote:
> On Wed, 24 Nov 2021 13:23:42 +0000 Matthew Wilcox <willy@infradead.org> wrote:
> 
>> On Wed, Nov 24, 2021 at 03:08:49AM +0000, cgel.zte@gmail.com wrote:
>>> From: chiminghao <chi.minghao@zte.com.cn>
>>>
>>> Fix the following coccinelle report:
>>> ./mm/memory_hotplug.c:2210:2-5:
>>> WARNING  Use BUG_ON instead of if condition followed by BUG.
>>
>> What coccinelle script is reporting this?
>>
>>> -	if (try_remove_memory(start, size))
>>> -		BUG();
>>> +	BUG_ON(try_remove_memory(start, size));
>>
>> I really, really, really do not like this.  For functions with
>> side-effects, this is bad style.  If it's a pure predicate, then
>> sure, but this is bad.
> 
> I don't like it either.  Yes, BUG() is special but it's such dangerous
> practice.  I'd vote to change coccinelle.
> 

Definitely! Or at least use a safer pattern/habit, with just a passive
variable in the BUG_ON() call, approximately like this:

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 852041f6be41..48bd5ff341e7 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -2201,13 +2201,12 @@ static int __ref try_remove_memory(u64 start, u64 size)
*/
void __remove_memory(u64 start, u64 size)
{
-
+       int ret = try_remove_memory(start, size);
/*
* trigger BUG() if some memory is not offlined prior to calling this
* function
*/
-       if (try_remove_memory(start, size))
-               BUG();
+       BUG_ON(ret);
}

/*

...and by the way, while going to type that, I immediately stumbled upon
another pre-existing case of this sort of thing, in try_remove_memory(),
which does this:

static int __ref try_remove_memory(u64 start, u64 size)
{
	struct vmem_altmap mhp_altmap = {};
	struct vmem_altmap *altmap = NULL;
	unsigned long nr_vmemmap_pages;
	int rc = 0, nid = NUMA_NO_NODE;

	BUG_ON(check_hotplug_memory_range(start, size));

...


thanks,
-- 
John Hubbard
NVIDIA

Re: [PATCH] mm: Use BUG_ON instead of if condition followed by BUG

[Matthew Wilcox]

On Wed, Nov 24, 2021 at 01:23:42PM +0000, Matthew Wilcox wrote:
> On Wed, Nov 24, 2021 at 03:08:49AM +0000, cgel.zte@gmail.com wrote:
> > From: chiminghao <chi.minghao@zte.com.cn>
> > 
> > Fix the following coccinelle report:
> > ./mm/memory_hotplug.c:2210:2-5:
> > WARNING  Use BUG_ON instead of if condition followed by BUG.
> 
> What coccinelle script is reporting this?

Maybe I found it?

scripts/coccinelle/misc/bugon.cocci:msg="WARNING: Use BUG_ON instead of if condition followed by BUG.\nPlease make sure the condition has no side effects (see conditional BUG_ON definition in include/asm-generic/bug.h)"

Julia, Michal, can we delete this script, please?  It's being abused.

> > -	if (try_remove_memory(start, size))
> > -		BUG();
> > +	BUG_ON(try_remove_memory(start, size));
> 
> I really, really, really do not like this.  For functions with
> side-effects, this is bad style.  If it's a pure predicate, then
> sure, but this is bad.
> 

Re: [PATCH] mm: Use BUG_ON instead of if condition followed by BUG

[Matthew Wilcox]

On Wed, Nov 24, 2021 at 02:45:59PM -0800, John Hubbard wrote:
> @@ -2201,13 +2201,12 @@ static int __ref try_remove_memory(u64 start, u64 size)
> */
> void __remove_memory(u64 start, u64 size)
> {
> -
> +       int ret = try_remove_memory(start, size);
> /*
> * trigger BUG() if some memory is not offlined prior to calling this
> * function
> */
> -       if (try_remove_memory(start, size))
> -               BUG();
> +       BUG_ON(ret);
> }

I'd rather leave it the way it is.  I don't see why the version you
propose is better.

> ...and by the way, while going to type that, I immediately stumbled upon
> another pre-existing case of this sort of thing, in try_remove_memory(),
> which does this:
> 
> static int __ref try_remove_memory(u64 start, u64 size)
> {
> 	struct vmem_altmap mhp_altmap = {};
> 	struct vmem_altmap *altmap = NULL;
> 	unsigned long nr_vmemmap_pages;
> 	int rc = 0, nid = NUMA_NO_NODE;
> 
> 	BUG_ON(check_hotplug_memory_range(start, size));

That needs to be fixed.

Re: [PATCH] mm: Use BUG_ON instead of if condition followed by BUG

[John Hubbard]

On 11/24/21 19:32, Matthew Wilcox wrote:
> On Wed, Nov 24, 2021 at 02:45:59PM -0800, John Hubbard wrote:
>> @@ -2201,13 +2201,12 @@ static int __ref try_remove_memory(u64 start, u64 size)
>> */
>> void __remove_memory(u64 start, u64 size)
>> {
>> -
>> +       int ret = try_remove_memory(start, size);
>> /*
>> * trigger BUG() if some memory is not offlined prior to calling this
>> * function
>> */
>> -       if (try_remove_memory(start, size))
>> -               BUG();
>> +       BUG_ON(ret);
>> }
> 
> I'd rather leave it the way it is.  I don't see why the version you
> propose is better.


In isolation, it's *not* better. It's only potentially useful in the
context of "code plus tools". That is to say, if the coccinelle change
request were rejected, then this provides a way forward that is not
worse than the existing code, and also works around the warning.


> 
>> ...and by the way, while going to type that, I immediately stumbled upon
>> another pre-existing case of this sort of thing, in try_remove_memory(),
>> which does this:
>>
>> static int __ref try_remove_memory(u64 start, u64 size)
>> {
>> 	struct vmem_altmap mhp_altmap = {};
>> 	struct vmem_altmap *altmap = NULL;
>> 	unsigned long nr_vmemmap_pages;
>> 	int rc = 0, nid = NUMA_NO_NODE;
>>
>> 	BUG_ON(check_hotplug_memory_range(start, size));
> 
> That needs to be fixed.


Yes it does. :) I pointed it out in hopes that Chiminghao might be inspired
to go find and fix some of these.


thanks,
-- 
John Hubbard
NVIDIA

Re: [PATCH] mm: Use BUG_ON instead of if condition followed by BUG

[Julia Lawall]

On Thu, 25 Nov 2021, Matthew Wilcox wrote:

> On Wed, Nov 24, 2021 at 01:23:42PM +0000, Matthew Wilcox wrote:
> > On Wed, Nov 24, 2021 at 03:08:49AM +0000, cgel.zte@gmail.com wrote:
> > > From: chiminghao <chi.minghao@zte.com.cn>
> > >
> > > Fix the following coccinelle report:
> > > ./mm/memory_hotplug.c:2210:2-5:
> > > WARNING  Use BUG_ON instead of if condition followed by BUG.
> >
> > What coccinelle script is reporting this?
>
> Maybe I found it?
>
> scripts/coccinelle/misc/bugon.cocci:msg="WARNING: Use BUG_ON instead of if condition followed by BUG.\nPlease make sure the condition has no side effects (see conditional BUG_ON definition in include/asm-generic/bug.h)"
>
> Julia, Michal, can we delete this script, please?  It's being abused.

OK

julia

>
> > > -	if (try_remove_memory(start, size))
> > > -		BUG();
> > > +	BUG_ON(try_remove_memory(start, size));
> >
> > I really, really, really do not like this.  For functions with
> > side-effects, this is bad style.  If it's a pure predicate, then
> > sure, but this is bad.
> >
>