linux-kernel - Re: [PATCH] Detect and warn on atomic_inc/atomic

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Date:	Thu, 30 Apr 2009 14:45:26 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Nikanth Karthikesan <knikanth@...ell.com>
Cc:	mingo@...e.hu, jens.axboe@...cle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Detect and warn on atomic_inc/atomic_dec wrapping
 around

On Thu, 30 Apr 2009 19:39:50 +0530
Nikanth Karthikesan <knikanth@...ell.com> wrote:

> 

If I had a dollar for each wordwrapped patch I get sent...

> Detect and warn on atomic_inc/atomic_dec overflow.
> 
> Add a debug option to detect and warn when the 32-bit atomic_t overflows
> during atomic_inc and atomic_dec.
> 

OK.  

I'll beef the changelog up a bit - this one is wimpy.

The question is: do we put this in mainline?  I guess we might as well
give it a shot.  It may well find bugs and it might also trigger false
positives.  We can then fix the bugs and decide whether the false
positives warrant reverting it again, all very easy.

> +#include <asm/bug.h>

checkpatch says

WARNING: Use #include <linux/bug.h> instead of <asm/bug.h>
#215: FILE: include/asm-generic/atomic.h:11:
+#include <asm/bug.h>

Was this an oversight, or did you try using linux/bug.h and discovered
some problem?

> index 812c282..773c1a4 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -17,6 +17,17 @@ config ENABLE_WARN_DEPRECATED
>  	  Disable this to suppress the "warning: 'foo' is deprecated
>  	  (declared at kernel/power/somefile.c:1234)" messages.
>  
> +config HAVE_ARCH_DEBUG_ATOMIC
> +	bool
> +
> +config ENABLE_WARN_ATOMIC_INC_WRAP
> +	bool "Enable warning on atomic_inc()/atomic_dec() wrap"
> +	depends on HAVE_ARCH_DEBUG_ATOMIC
> +	default y
> +	help
> +	  Enable printing a warning when atomic_inc() or atomic_dec()
> +	  operation wraps around the 32-bit value.
> +

Yes, I agree with `default y' for now.  But we might want to turn it
off again later.  Adding that WARN to every atomic_inc/atomic_dec site
must do terrible things to the kernel text footprint.

Of course, if we make if `default y' for a while and then switch it to
`default n', the `y' state will linger for a very long time in all the
kernel developers' .configs.  Good!  Very sneaky.
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