Re: [PATCH v2 2/4] cxl/region: Prevent device_find_child() from modifying caller's match data

[Zijun Hu <zijun_hu@icloud.com>]

On 2024/8/20 21:59, Ira Weiny wrote:
> Zijun Hu wrote:
>> From: Zijun Hu <quic_zijuhu@quicinc.com>
>>
>> To prepare for constifying the following old driver core API:
>>
>> struct device *device_find_child(struct device *dev, void *data,
>> 		int (*match)(struct device *dev, void *data));
>> to new:
>> struct device *device_find_child(struct device *dev, const void *data,
>> 		int (*match)(struct device *dev, const void *data));
>>
>> The new API does not allow its match function (*match)() to modify
>> caller's match data @*data, but match_free_decoder() as the old API's
>> match function indeed modifies relevant match data, so it is not
>> suitable for the new API any more, fixed by implementing a equivalent
>> cxl_device_find_child() instead of the old API usage.
> 
> Generally it seems ok but I think some name changes will make this more
> clear.  See below.
> 

okay.

> Also for those working on CXL I'm questioning the use of ID here and the
> dependence on the id's being added to the parent in order.  Is that a
> guarantee?
> 
>>
>> Signed-off-by: Zijun Hu <quic_zijuhu@quicinc.com>
>> ---
>>  drivers/cxl/core/region.c | 36 +++++++++++++++++++++++++++++++++++-
>>  1 file changed, 35 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
>> index 21ad5f242875..8d8f0637f7ac 100644
>> --- a/drivers/cxl/core/region.c
>> +++ b/drivers/cxl/core/region.c
>> @@ -134,6 +134,39 @@ static const struct attribute_group *get_cxl_region_access1_group(void)
>>  	return &cxl_region_access1_coordinate_group;
>>  }
>>  
>> +struct cxl_dfc_data {
> 
> struct cxld_match_data
> 
> 'cxld' == cxl decoder in our world.
> 

make sense.

>> +	int (*match)(struct device *dev, void *data);
>> +	void *data;
>> +	struct device *target_device;
>> +};
>> +
>> +static int cxl_dfc_match_modify(struct device *dev, void *data)
> 
> Why not just put this logic into match_free_decoder?
> 

Actually, i ever considered solution B as you suggested in the end.

For this change, namely, solution A:
1) this change is clearer and easier to understand.
2) this change does not touch any existing cxld logic

For solution B:
it is more reasonable

i finally select A since it can express my concern and relevant solution
clearly.

>> +{
>> +	struct cxl_dfc_data *dfc_data = data;
>> +	int res;
>> +
>> +	res = dfc_data->match(dev, dfc_data->data);
>> +	if (res && get_device(dev)) {
>> +		dfc_data->target_device = dev;
>> +		return res;
>> +	}
>> +
>> +	return 0;
>> +}
>> +
>> +/*
>> + * I have the same function as device_find_child() but allow to modify
>> + * caller's match data @*data.
>> + */
> 
> No need for this comment after the new API is established.
> 

i have given up the idea within v1 to introduce a new API which *should
ONLY* be used by this patch series, so it is not worthy of a new API
even if it can bring convenient for this patch series.

>> +static struct device *cxl_device_find_child(struct device *parent, void *data,
>> +					    int (*match)(struct device *dev, void *data))
>> +{
>> +	struct cxl_dfc_data dfc_data = {match, data, NULL};
>> +
>> +	device_for_each_child(parent, &dfc_data, cxl_dfc_match_modify);
>> +	return dfc_data.target_device;
>> +}
>> +
>>  static ssize_t uuid_show(struct device *dev, struct device_attribute *attr,
>>  			 char *buf)
>>  {
>> @@ -849,7 +882,8 @@ cxl_region_find_decoder(struct cxl_port *port,
>>  		dev = device_find_child(&port->dev, &cxlr->params,
>>  					match_auto_decoder);
>>  	else
>> -		dev = device_find_child(&port->dev, &id, match_free_decoder);
>> +		dev = cxl_device_find_child(&port->dev, &id,
>> +					    match_free_decoder);
> 
> This is too literal.  How about the following (passes basic cxl-tests).
> 

it is reasonable.

do you need me to submit that you suggest in the end and add you as
co-developer ?

OR

you submit it by yourself ?

either is okay for me.

> Ira
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c              
> index 21ad5f242875..c1e46254efb8 100644                                         
> --- a/drivers/cxl/core/region.c                                                 
> +++ b/drivers/cxl/core/region.c                                                 
> @@ -794,10 +794,15 @@ static size_t show_targetN(struct cxl_region *cxlr, char *buf, int pos)
>         return rc;                                                              
>  }                                                                              
>                                                                                 
> +struct cxld_match_data {                                                       
> +       int id;                                                                 
> +       struct device *target_device;                                           
> +};                                                                             
> +                                                                               
>  static int match_free_decoder(struct device *dev, void *data)                  
>  {                                                                              
> +       struct cxld_match_data *match_data = data;                              
>         struct cxl_decoder *cxld;                                               
> -       int *id = data;                                                         
>                                                                                 
>         if (!is_switch_decoder(dev))                                            
>                 return 0;                                                       
> @@ -805,17 +810,30 @@ static int match_free_decoder(struct device *dev, void *data)
>         cxld = to_cxl_decoder(dev);                                             
>                                                                                 
>         /* enforce ordered allocation */                                        
> -       if (cxld->id != *id)                                                    
> +       if (cxld->id != match_data->id)                                         
>                 return 0;                                                       
>                                                                                 
> -       if (!cxld->region)                                                      
> +       if (!cxld->region && get_device(dev)) {                                 

get_device(dev) failure may cause different logic against existing
but i think it should be impossible to happen normally.

> +               match_data->target_device = dev;                                
>                 return 1;                                                       
> +       }                                                                       
>                                                                                 
> -       (*id)++;                                                                
> +       match_data->id++;                                                       
>                                                                                 
>         return 0;                                                               
>  }                                                                              
>                                                                                 
> +static struct device *find_free_decoder(struct device *parent)                 
> +{                                                                              
> +       struct cxld_match_data match_data = {                                   
> +               .id = 0,                                                        
> +               .target_device = NULL,                                          
> +       };                                                                      
> +                                                                               
> +       device_for_each_child(parent, &match_data, match_free_decoder);         
> +       return match_data.target_device;                                        
> +}                                                                              
> +                                                                               
>  static int match_auto_decoder(struct device *dev, void *data)                  
>  {                                                                              
>         struct cxl_region_params *p = data;                                     
> @@ -840,7 +858,6 @@ cxl_region_find_decoder(struct cxl_port *port,              
>                         struct cxl_region *cxlr)                                
>  {                                                                              
>         struct device *dev;                                                     
> -       int id = 0;                                                             
>                                                                                 
>         if (port == cxled_to_port(cxled))                                       
>                 return &cxled->cxld;                                            
> @@ -849,7 +866,8 @@ cxl_region_find_decoder(struct cxl_port *port,              
>                 dev = device_find_child(&port->dev, &cxlr->params,              
>                                         match_auto_decoder);                    
>         else                                                                    
> -               dev = device_find_child(&port->dev, &id, match_free_decoder);   
> +               dev = find_free_decoder(&port->dev);                            
> +                                                                               
>         if (!dev)                                                               
>                 return NULL;                                                    
>         /*