Re: [patch 2.6.26-rc1] i2c_use_client() defends against NULL

[Jean Delvare <khali-PUYAD+kWke1g9hUCZPvPmw@public.gmane.org>]

Hi David,

On Sun, 4 May 2008 13:06:20 -0700, David Brownell wrote:
> Defend the i2c refcount calls against NULL pointers, as is important
> (and conventional) for such calls ... we don't want to morph NULL
> pointers into bogus non-null ones.

Passing NULL to the current functions would just crash, it wouldn't
morph anything.

>  Note that none of the current
> callers of i2c_use_client() use its return value.

Which sounds sane... when a function can only fail if you give it a
bogus parameter, it makes more sense to check the parameter for
validity yourself than to check the value returned by the function
afterwards. I'm not even sure why these functions return something in
the first place.

> 
> Signed-off-by: David Brownell <dbrownell-Rn4VEauK+AKRv+LV9MX5uipxlwaOVQ5f@public.gmane.org>
> ---
>  drivers/i2c/i2c-core.c |    8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> --- g26.orig/drivers/i2c/i2c-core.c	2008-05-04 12:50:33.000000000 -0700
> +++ g26/drivers/i2c/i2c-core.c	2008-05-04 12:51:04.000000000 -0700
> @@ -1013,8 +1013,9 @@ EXPORT_SYMBOL(i2c_detach_client);
>   */
>  struct i2c_client *i2c_use_client(struct i2c_client *client)
>  {
> -	get_device(&client->dev);
> -	return client;
> +	if (client && get_device(&client->dev))
> +		return client;
> +	return NULL;
>  }
>  EXPORT_SYMBOL(i2c_use_client);
>  
> @@ -1026,7 +1027,8 @@ EXPORT_SYMBOL(i2c_use_client);
>   */
>  void i2c_release_client(struct i2c_client *client)
>  {
> -	put_device(&client->dev);
> +	if (client)
> +		put_device(&client->dev);
>  }
>  EXPORT_SYMBOL(i2c_release_client);
>  

I hate this defensive programming, to me it's a waste of CPU cycles.
But as you said, it's apparently the norm in the Linux kernel to do
these useless checks for that specific type of functions, so I'll take
your patch even though I don't like it.

Thanks,
-- 
Jean Delvare

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