On Tue, Mar 06, 2012 at 04:08:33PM -0800, Andrew Morton wrote:

On Sun, 4 Mar 2012 13:43:32 -0800 (PST)
David Rientjes [off-list ref] wrote:

quoted

BUG() is a no-op when CONFIG_BUG is disabled, so slab_node() needs a
dummy return value to avoid reaching the end of a non-void function.

Signed-off-by: David Rientjes <rientjes@google.com>
---
 mm/mempolicy.c |    1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c

@@ -1611,6 +1611,7 @@ unsigned slab_node(struct mempolicy *policy)
 
 	default:
 		BUG();
+		return numa_node_id();
 	}
 }

Wait.  If the above code generated a warning then surely we get a *lot*
of warnings!  I'd expect that a lot of code assumes that BUG() never
returns?

In a quick make (ARCH=um defconfig | CONFIG_BUG=n) the following four
warnings have popped out: 

kernel/sched/core.c:3144:1: warning: control reaches end of non-void function
[-Wreturn-type]
mm/bootmem.c:352:1: warning: control reaches end of non-void function
[-Wreturn-type]
fs/locks.c:1469:1: warning: control reaches end of non-void function
[-Wreturn-type]
block/cfq-iosched.c:2912:1: warning: control reaches end of non-void function
[-Wreturn-type]
net/core/ethtool.c:211:1: warning: control reaches end of non-void function
[-Wreturn-type]


So, yes... Unfortunately, we would see a lot more warnings for a (more) complete
kernel configuration.

Can we fix this within the BUG() definition?  I can't think of a way,
unless gcc gives us a way of accessing the return type of the current
function, and I don't think it does that.


Also, does CONIG_BUG=n even make sense?  If we got here and we know
that the kernel has malfunctioned, what point is there in pretending
otherwise?  Odd.

I admit I was thinking about in follow David's example and start chasing
similar cases to propose a janitorial patch, however, I couldn't agree more with
your point here. It seems odd turning CONFIG_BUG off and neglect well known buggy
conditions within the code. Perhaps, then, the best way to cope with this oddity
would be just drop CONFIG_BUG config knob at all, making it permanently "on".

Any other thoughts?

  Rafael

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