linux-kernel - Re: [PATCH 13/14] tools lib fd array: Allow associating an integer cookie with each entry

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Message-ID: <20140911160711.GF32722@krava.brq.redhat.com>
Date:	Thu, 11 Sep 2014 18:07:11 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:	linux-kernel@...r.kernel.org,
	Adrian Hunter <adrian.hunter@...el.com>,
	Borislav Petkov <bp@...e.de>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Jean Pihet <jean.pihet@...aro.org>,
	Jiri Olsa <jolsa@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 13/14] tools lib fd array: Allow associating an integer
 cookie with each entry

On Thu, Sep 11, 2014 at 12:49:22PM -0300, Arnaldo Carvalho de Melo wrote:
> Em Thu, Sep 11, 2014 at 05:35:27PM +0200, Jiri Olsa escreveu:
> > On Thu, Sep 11, 2014 at 12:23:30PM -0300, Arnaldo Carvalho de Melo wrote:
> > > Em Thu, Sep 11, 2014 at 04:59:45PM +0200, Jiri Olsa escreveu:
> > > > On Thu, Sep 11, 2014 at 10:29:11AM -0300, Arnaldo Carvalho de Melo wrote:
> > > > > But then I had no use whatsoever for the ->ptr one at this point, so I
> > > > > just nuked it, to keep _just_ what is used _right now_, and added the
> > > > > comment to the changelog :-)
> 
> > > > if we are treeting tools/lib/api as 'external' lib, I think we should
> > > > use 'void *' for priv and let the user retype it to whatever he wants
> 
> > > > but I dont care/insist here too much.. it just seems strange to me ;-)
> > > 
> > > Ok, so perhaps it should be:
> > > 
> > > 	struct fdarray {
> > > 		...
> > > 		union {
> > > 			int idx;
> > > 		} *priv;
> > > 		...
> > > }
> > > 
> > > And then perf's evlist will use:
> > > 
> > > 	fda->priv[fd].idx;
> > > 
> > > 	Then, when we get the need for a pointer, we can go there and
> > > add it to the union.
> > > 
> > > 	This will make tools/lib/api/fd/ a little bit more friendly to
> > > use outside the kernel sources, since then a new version will be source
> > > code compatible (but not necessarily binary compatible).
> > 
> > you lost me ;-)
> > 
> > could you please make an example of the !compatibility you mentioned?
> 
> If I leave it as is now, when I add that void * pointer to each entry in
> fdarray->priv[], I will have to change fdarray's users, i.e. right now I
> will have to go and change evlist.h, i.e. every place that has:
> 
> 	fda->priv[fd];
> 
> 	will have to be changed to:
> 
> 	fda->priv[fd].idx;
> 
> If I instead change it to:
> 
>  	struct fdarray {
>  		...
>  		union {
>  			int idx;
>  		} *priv;
>  		...
> 	}
> 
> Then I will not have to change anything when I add that void * pointer
> to the union.

I dont think there's any trouble like that if you keep
just void* priv pointer and let users to store there
whatever they like

>  
> > > 	If we commit to that we can't just go on and change all its
> > > users as we can now do for things like 'struct sk_buff', 'struct sock'
> > > and any other kernel data structures.
> > > 	There are costs in making it 'external' in the sense you're
> > > implying. I want to make it a bit less 'external' (in your sense), at
> > > least while we initially implement new stuff like this one.
>  
> > and the costs?
>  
> > also I did not mean that union above, what I meant was:
>  
> > struct fdarray {
> > 	...
> > 	void *priv;
> > 	...
> > }
> 
> Yeah, that is what I am trying to avoid. This will make it use
> sizeof(void *) for each entry, and I have no use for that right now. All
> I need is sizeof(int) for each entry.

hm? but you're using pointer (int*) not int .. size will be sizeof(unsigned long) anyway

jirka
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