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

[Ira Weiny <ira.weiny@intel.com>]

Zijun Hu wrote:
> 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.

Understood.

> 
> >> +{
> >> +	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.

I'm not clear on this.  Are you still proposing to change the parameter to
const?

> 
> >> +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 ?

You can submit it with Suggested-by:

> 
> 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.

Indeed this is slightly different.  :-/

Move the get_device() to find_free_decoder()?

Ira

> 
> > +               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;                                        
> > +}                                                                              
> > +                                                                               

[snip]