netdev - Re: [PATCH v2 1/4] driver core: Make parameter check consistent for API cluster device_(for_each|find)

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Message-ID: <66c8c4a0633e9_a87cd294f6@iweiny-mobl.notmuch>
Date: Fri, 23 Aug 2024 12:19:28 -0500
From: Ira Weiny <ira.weiny@...el.com>
To: Zijun Hu <zijun_hu@...oud.com>, Ira Weiny <ira.weiny@...el.com>, "Greg
 Kroah-Hartman" <gregkh@...uxfoundation.org>, "Rafael J. Wysocki"
	<rafael@...nel.org>, Davidlohr Bueso <dave@...olabs.net>, Jonathan Cameron
	<jonathan.cameron@...wei.com>, Dave Jiang <dave.jiang@...el.com>, "Alison
 Schofield" <alison.schofield@...el.com>, Vishal Verma
	<vishal.l.verma@...el.com>, Dan Williams <dan.j.williams@...el.com>, "Takashi
 Sakamoto" <o-takashi@...amocchi.jp>, Timur Tabi <timur@...nel.org>, "David S.
 Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, "Jakub
 Kicinski" <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>
CC: <linux-kernel@...r.kernel.org>, <linux-cxl@...r.kernel.org>,
	<linux1394-devel@...ts.sourceforge.net>, <netdev@...r.kernel.org>, Zijun Hu
	<quic_zijuhu@...cinc.com>
Subject: Re: [PATCH v2 1/4] driver core: Make parameter check consistent for
 API cluster device_(for_each|find)_child()

Zijun Hu wrote:
> On 2024/8/20 22:14, Ira Weiny wrote:
> > Zijun Hu wrote:
> >> On 2024/8/20 20:53, Ira Weiny wrote:
> >>> Zijun Hu wrote:
> >>>> From: Zijun Hu <quic_zijuhu@...cinc.com>
> >>>>
> >>>> The following API cluster takes the same type parameter list, but do not
> >>>> have consistent parameter check as shown below.
> >>>>
> >>>> device_for_each_child(struct device *parent, ...)  // check (!parent->p)
> >>>> device_for_each_child_reverse(struct device *parent, ...) // same as above
> >>>> device_find_child(struct device *parent, ...)      // check (!parent)
> >>>>
> >>>
> >>> Seems reasonable.
> >>>
> >>> What about device_find_child_by_name()?
> >>>
> >>
> >> Plan to simplify this API implementation by * atomic * API
> >> device_find_child() as following:
> >>
> >> https://lore.kernel.org/all/20240811-simply_api_dfcbn-v2-1-d0398acdc366@quicinc.com
> >> struct device *device_find_child_by_name(struct device *parent,
> >>  					 const char *name)
> >> {
> >> 	return device_find_child(parent, name, device_match_name);
> >> }
> > 
> > Ok.  Thanks.
> > 
> >>
> >>>> Fixed by using consistent check (!parent || !parent->p) for the cluster.
> >>>>
> >>>> Signed-off-by: Zijun Hu <quic_zijuhu@...cinc.com>
> >>>> ---
> >>>>  drivers/base/core.c | 6 +++---
> >>>>  1 file changed, 3 insertions(+), 3 deletions(-)
> >>>>
> >>>> diff --git a/drivers/base/core.c b/drivers/base/core.c
> >>>> index 1688e76cb64b..b1dd8c5590dc 100644
> >>>> --- a/drivers/base/core.c
> >>>> +++ b/drivers/base/core.c
> >>>> @@ -4004,7 +4004,7 @@ int device_for_each_child(struct device *parent, void *data,
> >>>>  	struct device *child;
> >>>>  	int error = 0;
> >>>>  
> >>>> -	if (!parent->p)
> >>>> +	if (!parent || !parent->p)
> >>>>  		return 0;
> >>>>  
> >>>>  	klist_iter_init(&parent->p->klist_children, &i);
> >>>> @@ -4034,7 +4034,7 @@ int device_for_each_child_reverse(struct device *parent, void *data,
> >>>>  	struct device *child;
> >>>>  	int error = 0;
> >>>>  
> >>>> -	if (!parent->p)
> >>>> +	if (!parent || !parent->p)
> >>>>  		return 0;
> >>>>  
> >>>>  	klist_iter_init(&parent->p->klist_children, &i);
> >>>> @@ -4068,7 +4068,7 @@ struct device *device_find_child(struct device *parent, void *data,
> >>>>  	struct klist_iter i;
> >>>>  	struct device *child;
> >>>>  
> >>>> -	if (!parent)
> >>>> +	if (!parent || !parent->p)
> >>>
> >>> Perhaps this was just a typo which should have been.
> >>>
> >>> 	if (!parent->p)
> >>> ?
> >>>
> >> maybe, but the following device_find_child_by_name() also use (!parent).
> >>
> >>> I think there is an expectation that none of these are called with a NULL
> >>> parent.
> >>>
> >>
> >> this patch aim is to make these atomic APIs have consistent checks as
> >> far as possible, that will make other patches within this series more
> >> acceptable.
> >>
> >> i combine two checks to (!parent || !parent->p) since i did not know
> >> which is better.
> > 
> > I'm not entirely clear either.  But checking the member p makes more sense
> > to me than the parent parameter.  I would expect that iterating the
> > children of a device must be done only when the parent device is not NULL.
> > 
> > parent->p is more subtle.  I'm unclear why the API would need to allow
> > that to run without error.
> > 
> i prefer (!parent || !parent->p) with below reasons:
> 
> 1)
> original API authors have such concern that either (!parent) or
> (!parent->p) maybe happen since they are checked, all their concerns
> can be covered by (!parent || !parent->p).
> 
> 2)
> It is the more robust than either (!parent) or (!parent->p)
> 
> 3)
> it also does not have any negative effect.

It adds code and instructions to all paths calling these functions.

What is the reason to allow?

void foo() {
...
	device_for_each_child(NULL, ...);
...
}

What are we finding the child of in that case?

Ira

> 
> > Ira
>