question about lists

question about lists

[Julia Lawall]

Hello,

I was wondering what is the point of code like the following:

        INIT_LIST_HEAD(&bb->list);
        list_add(&bb->list, &s->workload->shadow_bb);

As far as I can see, list_add will initialize both fields of the list_head
structure without looking at their values, so why is the INIT_LIST_HEAD
needed?

thanks,
julia

Re: question about lists

[Dan Carpenter]

On Sun, May 03, 2020 at 11:10:01PM +0200, Julia Lawall wrote:
> Hello,
> 
> I was wondering what is the point of code like the following:
> 
>         INIT_LIST_HEAD(&bb->list);
>         list_add(&bb->list, &s->workload->shadow_bb);
> 
> As far as I can see, list_add will initialize both fields of the list_head
> structure without looking at their values, so why is the INIT_LIST_HEAD
> needed?

Yeah.  You're right.  It's not needed.

regards,
dan carpenter

Re: question about lists

[Julia Lawall]

On Mon, 4 May 2020, Dan Carpenter wrote:

> On Sun, May 03, 2020 at 11:10:01PM +0200, Julia Lawall wrote:
> > Hello,
> >
> > I was wondering what is the point of code like the following:
> >
> >         INIT_LIST_HEAD(&bb->list);
> >         list_add(&bb->list, &s->workload->shadow_bb);
> >
> > As far as I can see, list_add will initialize both fields of the list_head
> > structure without looking at their values, so why is the INIT_LIST_HEAD
> > needed?
>
> Yeah.  You're right.  It's not needed.

OK, there seem to be a good number of them, so I wondered if I missed
something.

thanks,
julia

Re: question about lists

[Christophe JAILLET]

Le 03/05/2020 à 23:10, Julia Lawall a écrit :
> Hello,
>
> I was wondering what is the point of code like the following:
>
>          INIT_LIST_HEAD(&bb->list);
>          list_add(&bb->list, &s->workload->shadow_bb);
>
> As far as I can see, list_add will initialize both fields of the list_head
> structure without looking at their values, so why is the INIT_LIST_HEAD
> needed?
>
> thanks,
> julia
>
Hi,

I agree with you. I don't see the need of 'INIT_LIST_HEAD' here.

The only reasons I could see are:
    - initializing an "object" looks a safe thing to do before using it, 
even if useless.
    - in your example, before commit f52c380a48f52, it was allocated 
with kmalloc, so it was maybe to avoid a really unlikely 
'__list_add_valid' failure. Now that it is kzalloc'ed, this can not 
happen anymore.

Just my 2c.

CJ

Re: question about lists

[Dan Carpenter]

On Mon, May 04, 2020 at 09:41:21PM +0200, Christophe JAILLET wrote:
>    - in your example, before commit f52c380a48f52, it was allocated with
> kmalloc, so it was maybe to avoid a really unlikely '__list_add_valid'
> failure. Now that it is kzalloc'ed, this can not happen anymore.

No, it was never possible to get a __list_add_valid() failure.  The list
you're adding to has to be initialized, but the list item that doesn't.
In __list_add_valid() the "new" is "&entry_obj->list" which is an offset
into the entry_obj object.

regards,
dan carpenter