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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, Aug 20, 2024 at 08:59:18AM -0500, 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.
> 
> 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.
> 
> > +	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?
> 
> > +{
> > +	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.
> 
> > +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).
> 
> 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)) {                                 
> +               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;                                        
> +}                                                                              

I like this type of re-write much better, thanks!

greg k-h