2021-03-17 02:47:27

by Jiapeng Chong

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

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 <[email protected]>
Signed-off-by: Jiapeng Chong <[email protected]>
---
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

2021-03-17 08:41:37

by Ingo Molnar

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

* Jiapeng Chong <[email protected]> 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 <[email protected]>
> Signed-off-by: Jiapeng Chong <[email protected]>
> ---
> 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

2021-03-17 08:43:29

by Ingo Molnar

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

* Ingo Molnar <[email protected]> wrote:

>
> * Jiapeng Chong <[email protected]> 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 <[email protected]>
> > Signed-off-by: Jiapeng Chong <[email protected]>
> > ---
> > 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

2021-03-17 12:56:59

by Arnd Bergmann

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

On Wed, Mar 17, 2021 at 9:45 AM Ingo Molnar <[email protected]> wrote:
> * Ingo Molnar <[email protected]> wrote:
> > * Jiapeng Chong <[email protected]> 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 <[email protected]>
> > > Signed-off-by: Jiapeng Chong <[email protected]>
> > > ---
> > > 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

2021-03-17 21:45:03

by Ingo Molnar

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

* Arnd Bergmann <[email protected]> wrote:

> On Wed, Mar 17, 2021 at 9:45 AM Ingo Molnar <[email protected]> wrote:
> > * Ingo Molnar <[email protected]> wrote:
> > > * Jiapeng Chong <[email protected]> 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 <[email protected]>
> > > > Signed-off-by: Jiapeng Chong <[email protected]>
> > > > ---
> > > > 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

2021-03-17 21:51:14

by Arnd Bergmann

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

On Wed, Mar 17, 2021 at 9:05 PM Ingo Molnar <[email protected]> wrote:
> * Arnd Bergmann <[email protected]> wrote:
> > On Wed, Mar 17, 2021 at 9:45 AM Ingo Molnar <[email protected]> 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