Re: [RFC PATCH] notifier: Return non-null when callback already registered

[Peter Zijlstra]

On Thu, May 20, 2021 at 10:20:33PM +0200, Borislav Petkov wrote:
> From: Borislav Petkov <bp@xxxxxxx>
> 
> The notifier registration routine doesn't return a proper error value
> when a callback has already been registered, leading people to track
> whether that regisration has happened at the call site:
> 
>   https://lore.kernel.org/amd-gfx/20210512013058.6827-1-mukul.joshi@xxxxxxx/
> 
> Which is unnecessary.
> 
> Return a non-null to signal that case so that callers can act
> accordingly.
> 
> Signed-off-by: Borislav Petkov <bp@xxxxxxx>
> ---
>  kernel/notifier.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/notifier.c b/kernel/notifier.c
> index 1b019cbca594..ff7a3198c5fc 100644
> --- a/kernel/notifier.c
> +++ b/kernel/notifier.c
> @@ -25,7 +25,7 @@ static int notifier_chain_register(struct notifier_block **nl,
>  	while ((*nl) != NULL) {
>  		if (unlikely((*nl) == n)) {
>  			WARN(1, "double register detected");

That should give a big clue^

> -			return 0;
> +			return 1;

How about -EBUSY here?