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[PATCH] sched/core: switch to BUG_ON()

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  [PATCH] sched/core: switch to BUG_ON() Nicholas Mc Guire <der.herr@hofr.at> - 2017-03-27 09:50 +0200
    Re: [PATCH] sched/core: switch to BUG_ON() Peter Zijlstra <peterz@infradead.org> - 2017-03-27 10:30 +0200
      Re: [PATCH] sched/core: switch to BUG_ON() Nicholas Mc Guire <der.herr@hofr.at> - 2017-03-27 11:20 +0200

#1609536 — [PATCH] sched/core: switch to BUG_ON()

Use BUG_ON() rather than an explicit if followed by BUG().

Signed-off-by: Nicholas Mc Guire <der.herr@hofr.at>
---

Found by coccinelle: bugon.cocci
./kernel/sched/core.c:5913:2-5: WARNING: Use BUG_ON instead of if condition followed by BUG.

There is one checkpatch.pl warning but thats a false positive. To panik()
when there is no core to run init on sounds resonable.

Patch was compile-tested with: x86_64_defconfig (implies CONFIG_SMP=y)
(some unrelated sparse warnings)

Patch is against 4.11-rc4 (localversion-next is next-20170327)

 kernel/sched/core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ac88348..b732c16 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5909,8 +5909,8 @@ 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, non_isolated_cpus) < 0)
-		BUG();
+	BUG_ON(set_cpus_allowed_ptr(current, non_isolated_cpus) < 0);
+
 	sched_init_granularity();
 	free_cpumask_var(non_isolated_cpus);
 
-- 
2.1.4

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#1609565

On Mon, Mar 27, 2017 at 09:40:46AM +0200, Nicholas Mc Guire wrote:

> Found by coccinelle: bugon.cocci
> ./kernel/sched/core.c:5913:2-5: WARNING: Use BUG_ON instead of if condition followed by BUG.

Right, so I disagree with that here.

> @@ -5909,8 +5909,8 @@ 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, non_isolated_cpus) < 0)
> -		BUG();
> +	BUG_ON(set_cpus_allowed_ptr(current, non_isolated_cpus) < 0);
> +
>  	sched_init_granularity();
>  	free_cpumask_var(non_isolated_cpus);

If the condition were without side effects, that pattern would hold, but
here the function call set_cpus_allowed_ptr() is very much not a pure
function, so wrapping it in a BUG_ON() just fels wrong.

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#1609599

On Mon, Mar 27, 2017 at 10:24:40AM +0200, Peter Zijlstra wrote:
> On Mon, Mar 27, 2017 at 09:40:46AM +0200, Nicholas Mc Guire wrote:
> 
> > Found by coccinelle: bugon.cocci
> > ./kernel/sched/core.c:5913:2-5: WARNING: Use BUG_ON instead of if condition followed by BUG.
> 
> Right, so I disagree with that here.
> 
> > @@ -5909,8 +5909,8 @@ 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, non_isolated_cpus) < 0)
> > -		BUG();
> > +	BUG_ON(set_cpus_allowed_ptr(current, non_isolated_cpus) < 0);
> > +
> >  	sched_init_granularity();
> >  	free_cpumask_var(non_isolated_cpus);
> 
> If the condition were without side effects, that pattern would hold, but
> here the function call set_cpus_allowed_ptr() is very much not a pure
> function, so wrapping it in a BUG_ON() just fels wrong.

Got it - just looked throug core.c and there it seems only side-effect
free test-functions are wrapped in BUG_ON() so this change is inconsistent.
Other subsystems seem not to follow that logic - might be worth a note
somewhere below Documentation/.

thx!
hofrat

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