Re: [Containers] [PATCH 5/7] pid: Implement pid_nr

[Dave Hansen <haveblue@us.ibm.com>]

On Tue, 2006-08-15 at 13:00 -0600, Eric W. Biederman wrote:
> Dave Hansen <haveblue@us.ibm.com> writes:
> 
> > On Tue, 2006-08-15 at 12:23 -0600, Eric W. Biederman wrote:
> >> +static inline pid_t pid_nr(struct pid *pid)
> >> +{
> >> +       pid_t nr = 0;
> >> +       if (pid)
> >> +               nr = pid->nr;
> >> +       return nr;
> >> +} 
> >
> > When is it valid to be passing around a NULL 'struct pid *'?
> 
> When you don't have one at all.  Look at the fcntl case a few
> patches later, or even the spawnpid case.

Does the fcntl() one originate from anywhere other than find_pid() in
f_setown()?  It seems like, perhaps, the error checking is being done at
the wrong level.

> Then of course there is the later chaos when we get to pid spaces
> where depending on the pid namespace you are in when you call this
> on a given struct pid sometimes you will get a pid value and sometimes
> you won't.

OK, I think it is makes sense to me to say 'get_pid(tsk, pidspace)' and
get back a NULL 'struct pid' if that task isn't visible in that
namespace.  However, I don't get how it is handy to be able to defer the
fact that the pid wasn't found until you go do a pid_nr() on that NULL.

-- Dave