[PATCH] sched: replace if (cond) BUG() with BUG_ON()

[PATCH] sched: replace if (cond) BUG() with BUG_ON()

[Jiapeng Chong]

Fix the following coccicheck warnings:

./kernel/sched/core.c:8039:2-5: WARNING: Use BUG_ON instead of if
condition followed by BUG.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
---
 kernel/sched/core.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9819121..7392bc0 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8035,8 +8035,7 @@ void __init sched_init_smp(void)
 	mutex_unlock(&sched_domains_mutex);
 
 	/* Move init over to a non-isolated CPU */
-	if (set_cpus_allowed_ptr(current, housekeeping_cpumask(HK_FLAG_DOMAIN)) < 0)
-		BUG();
+	BUG(set_cpus_allowed_ptr(current, housekeeping_cpumask(HK_FLAG_DOMAIN)) < 0);
 	sched_init_granularity();
 
 	init_sched_rt_class();
-- 
1.8.3.1

Re: [PATCH] sched: replace if (cond) BUG() with BUG_ON()

[Ingo Molnar]

* Jiapeng Chong <jiapeng.chong@linux.alibaba.com> wrote:

> Fix the following coccicheck warnings:
> 
> ./kernel/sched/core.c:8039:2-5: WARNING: Use BUG_ON instead of if
> condition followed by BUG.
> 
> Reported-by: Abaci Robot <abaci@linux.alibaba.com>
> Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
> ---
>  kernel/sched/core.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 9819121..7392bc0 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8035,8 +8035,7 @@ void __init sched_init_smp(void)
>  	mutex_unlock(&sched_domains_mutex);
>  
>  	/* Move init over to a non-isolated CPU */
> -	if (set_cpus_allowed_ptr(current, housekeeping_cpumask(HK_FLAG_DOMAIN)) < 0)
> -		BUG();
> +	BUG(set_cpus_allowed_ptr(current, housekeeping_cpumask(HK_FLAG_DOMAIN)) < 0);

The patch doesn't quite do what the title & changelog claims...

Thanks,

	Ingo

Re: [PATCH] sched: replace if (cond) BUG() with BUG_ON()

[Ingo Molnar]

* Ingo Molnar <mingo@kernel.org> wrote:

> 
> * Jiapeng Chong <jiapeng.chong@linux.alibaba.com> wrote:
> 
> > Fix the following coccicheck warnings:
> > 
> > ./kernel/sched/core.c:8039:2-5: WARNING: Use BUG_ON instead of if
> > condition followed by BUG.
> > 
> > Reported-by: Abaci Robot <abaci@linux.alibaba.com>
> > Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
> > ---
> >  kernel/sched/core.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 9819121..7392bc0 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -8035,8 +8035,7 @@ void __init sched_init_smp(void)
> >  	mutex_unlock(&sched_domains_mutex);
> >  
> >  	/* Move init over to a non-isolated CPU */
> > -	if (set_cpus_allowed_ptr(current, housekeeping_cpumask(HK_FLAG_DOMAIN)) < 0)
> > -		BUG();
> > +	BUG(set_cpus_allowed_ptr(current, housekeeping_cpumask(HK_FLAG_DOMAIN)) < 0);
> 
> The patch doesn't quite do what the title & changelog claims...

More importantly, we use this pattern when we don't want !CONFIG_BUG 
to remove the 'condition'.

I.e. the "side effect" here is important scheduler logic. It must 
never be optimized out.

Thanks,

	Ingo

Re: [PATCH] sched: replace if (cond) BUG() with BUG_ON()

[Arnd Bergmann]

On Wed, Mar 17, 2021 at 9:45 AM Ingo Molnar <mingo@kernel.org> wrote:
> * Ingo Molnar <mingo@kernel.org> wrote:
> > * Jiapeng Chong <jiapeng.chong@linux.alibaba.com> wrote:
> >
> > > Fix the following coccicheck warnings:
> > >
> > > ./kernel/sched/core.c:8039:2-5: WARNING: Use BUG_ON instead of if
> > > condition followed by BUG.
> > >
> > > Reported-by: Abaci Robot <abaci@linux.alibaba.com>
> > > Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
> > > ---
> > >  kernel/sched/core.c | 3 +--
> > >  1 file changed, 1 insertion(+), 2 deletions(-)
> > >
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index 9819121..7392bc0 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -8035,8 +8035,7 @@ void __init sched_init_smp(void)
> > >     mutex_unlock(&sched_domains_mutex);
> > >
> > >     /* Move init over to a non-isolated CPU */
> > > -   if (set_cpus_allowed_ptr(current, housekeeping_cpumask(HK_FLAG_DOMAIN)) < 0)
> > > -           BUG();
> > > +   BUG(set_cpus_allowed_ptr(current, housekeeping_cpumask(HK_FLAG_DOMAIN)) < 0);
> >
> > The patch doesn't quite do what the title & changelog claims...
>
> More importantly, we use this pattern when we don't want !CONFIG_BUG
> to remove the 'condition'.
>
> I.e. the "side effect" here is important scheduler logic. It must
> never be optimized out.

This behavior for !CONFIG_BUG has changed a while ago, it is now safe
to rely on the side-effect of the BUG_ON() condition regardless of
CONFIG_BUG. When that option is disabled, running into the condition
just ends up in a "do {} while (1)" loop.

        Arnd

Re: [PATCH] sched: replace if (cond) BUG() with BUG_ON()

[Ingo Molnar]

* Arnd Bergmann <arnd@kernel.org> wrote:

> On Wed, Mar 17, 2021 at 9:45 AM Ingo Molnar <mingo@kernel.org> wrote:
> > * Ingo Molnar <mingo@kernel.org> wrote:
> > > * Jiapeng Chong <jiapeng.chong@linux.alibaba.com> wrote:
> > >
> > > > Fix the following coccicheck warnings:
> > > >
> > > > ./kernel/sched/core.c:8039:2-5: WARNING: Use BUG_ON instead of if
> > > > condition followed by BUG.
> > > >
> > > > Reported-by: Abaci Robot <abaci@linux.alibaba.com>
> > > > Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
> > > > ---
> > > >  kernel/sched/core.c | 3 +--
> > > >  1 file changed, 1 insertion(+), 2 deletions(-)
> > > >
> > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > > index 9819121..7392bc0 100644
> > > > --- a/kernel/sched/core.c
> > > > +++ b/kernel/sched/core.c
> > > > @@ -8035,8 +8035,7 @@ void __init sched_init_smp(void)
> > > >     mutex_unlock(&sched_domains_mutex);
> > > >
> > > >     /* Move init over to a non-isolated CPU */
> > > > -   if (set_cpus_allowed_ptr(current, housekeeping_cpumask(HK_FLAG_DOMAIN)) < 0)
> > > > -           BUG();
> > > > +   BUG(set_cpus_allowed_ptr(current, housekeeping_cpumask(HK_FLAG_DOMAIN)) < 0);
> > >
> > > The patch doesn't quite do what the title & changelog claims...
> >
> > More importantly, we use this pattern when we don't want !CONFIG_BUG
> > to remove the 'condition'.
> >
> > I.e. the "side effect" here is important scheduler logic. It must
> > never be optimized out.
> 
> This behavior for !CONFIG_BUG has changed a while ago, it is now safe
> to rely on the side-effect of the BUG_ON() condition regardless of
> CONFIG_BUG. When that option is disabled, running into the condition
> just ends up in a "do {} while (1)" loop.

Dunno, I still think it's not a particularly clean pattern to 'hide' 
significant side effects within a BUG_ON().

Thanks,

	Ingo

Re: [PATCH] sched: replace if (cond) BUG() with BUG_ON()

[Arnd Bergmann]

On Wed, Mar 17, 2021 at 9:05 PM Ingo Molnar <mingo@kernel.org> wrote:
> * Arnd Bergmann <arnd@kernel.org> wrote:
> > On Wed, Mar 17, 2021 at 9:45 AM Ingo Molnar <mingo@kernel.org> wrote:
> > >
> > > More importantly, we use this pattern when we don't want !CONFIG_BUG
> > > to remove the 'condition'.
> > >
> > > I.e. the "side effect" here is important scheduler logic. It must
> > > never be optimized out.
> >
> > This behavior for !CONFIG_BUG has changed a while ago, it is now safe
> > to rely on the side-effect of the BUG_ON() condition regardless of
> > CONFIG_BUG. When that option is disabled, running into the condition
> > just ends up in a "do {} while (1)" loop.
>
> Dunno, I still think it's not a particularly clean pattern to 'hide'
> significant side effects within a BUG_ON().

Fair enough. Readability really is the key here, and I agree the current
version is easier to understand. The only architectures that even define
an optimized BUG_ON() are mips and powerpc, and saving a few cycles
is barely worth it in a fast path, which this is clearly not.

       Arnd

Re: [PATCH] sched: replace if (cond) BUG() with BUG_ON()

[Christophe Leroy]

Le 17/03/2021 à 07:45, Jiapeng Chong a écrit :
> Fix the following coccicheck warnings:
> 
> ./arch/powerpc/platforms/cell/spufs/sched.c:908:2-5: WARNING: Use BUG_ON
> instead of if condition followed by BUG.

Consider using WARN_ON() instead of BUG_ON() if relevant. If not, explain in the commit message why 
we need to keep a BUG_ON().

See https://www.kernel.org/doc/html/latest/process/deprecated.html#bug-and-bug-on


> 
> Reported-by: Abaci Robot <abaci@linux.alibaba.com>
> Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
> ---
>   arch/powerpc/platforms/cell/spufs/sched.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/platforms/cell/spufs/sched.c b/arch/powerpc/platforms/cell/spufs/sched.c
> index 3692064..139a6ec 100644
> --- a/arch/powerpc/platforms/cell/spufs/sched.c
> +++ b/arch/powerpc/platforms/cell/spufs/sched.c
> @@ -904,8 +904,7 @@ static noinline void spusched_tick(struct spu_context *ctx)
>   	struct spu_context *new = NULL;
>   	struct spu *spu = NULL;
>   
> -	if (spu_acquire(ctx))
> -		BUG();	/* a kernel thread never has signals pending */
> +	BUG_ON(spu_acquire(ctx));	/* a kernel thread never has signals pending */
>   
>   	if (ctx->state != SPU_STATE_RUNNABLE)
>   		goto out;
> 

[PATCH] sched: replace if (cond) BUG() with BUG_ON()

[Jiapeng Chong]

Fix the following coccicheck warnings:

./arch/powerpc/platforms/cell/spufs/sched.c:908:2-5: WARNING: Use BUG_ON
instead of if condition followed by BUG.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
---
 arch/powerpc/platforms/cell/spufs/sched.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/powerpc/platforms/cell/spufs/sched.c b/arch/powerpc/platforms/cell/spufs/sched.c
index 3692064..139a6ec 100644
--- a/arch/powerpc/platforms/cell/spufs/sched.c
+++ b/arch/powerpc/platforms/cell/spufs/sched.c
@@ -904,8 +904,7 @@ static noinline void spusched_tick(struct spu_context *ctx)
 	struct spu_context *new = NULL;
 	struct spu *spu = NULL;
 
-	if (spu_acquire(ctx))
-		BUG();	/* a kernel thread never has signals pending */
+	BUG_ON(spu_acquire(ctx));	/* a kernel thread never has signals pending */
 
 	if (ctx->state != SPU_STATE_RUNNABLE)
 		goto out;
-- 
1.8.3.1