Re: [PATCH] do_fork(): Rename 'stack_size' argument to reflect actual use

[Alex Dowad]

On 05/03/15 22:29, David Rientjes wrote:
> On Thu, 5 Mar 2015, Alex Dowad wrote:
>
> > > > diff --git a/kernel/fork.c b/kernel/fork.c
> > > > index cf65139..b38a2ae 100644
> > > > --- a/kernel/fork.c
> > > > +++ b/kernel/fork.c
> > > > @@ -1186,10 +1186,12 @@ init_task_pid(struct task_struct *task, enum
> > > > pid_type type, struct pid *pid)
> > > >     * It copies the registers, and all the appropriate
> > > >     * parts of the process environment (as per the clone
> > > >     * flags). The actual kick-off is left to the caller.
> > > > + *
> > > > + * When copying a kernel thread, 'stack_start' is the function to run.
> > > >     */
> > > >    static struct task_struct *copy_process(unsigned long clone_flags,
> > > >    					unsigned long stack_start,
> > > > -					unsigned long stack_size,
> > > > +					unsigned long kthread_arg,
> > > >    					int __user *child_tidptr,
> > > >    					struct pid *pid,
> > > >    					int trace)
> > > > @@ -1401,7 +1403,7 @@ static struct task_struct *copy_process(unsigned
> > > > long clone_flags,
> > > >    	retval = copy_io(clone_flags, p);
> > > >    	if (retval)
> > > >    		goto bad_fork_cleanup_namespaces;
> > > > -	retval = copy_thread(clone_flags, stack_start, stack_size, p);
> > > > +	retval = copy_thread(clone_flags, stack_start, kthread_arg, p);
> > > >    	if (retval)
> > > >    		goto bad_fork_cleanup_io;
> > > >    @@ -1629,8 +1631,8 @@ struct task_struct *fork_idle(int cpu)
> > > >     * it and waits for it to finish using the VM if required.
> > > >     */
> > > >    long do_fork(unsigned long clone_flags,
> > > > -	      unsigned long stack_start,
> > > > -	      unsigned long stack_size,
> > > > +	      unsigned long stack_start, /* or function for kthread to run */
> > > > +	      unsigned long kthread_arg,
> > > >    	      int __user *parent_tidptr,
> > > >    	      int __user *child_tidptr)
> > > >    {
> > > Looks fine, but I'm not sure about commenting functional formals.  Since
> > > copy_process() and do_fork() can have formals with different meanings,
> > > then why not just rename them "arg1" and "arg2" respectively and then
> > > define in the comment above the function what the possible combinations
> > > are?
> > The second argument is *only* ever used for one thing: an argument passed to a
> > kernel thread. That's why I would like to rename it to "kthread_arg". The
> > previous argument (currently named "stack_start") is indeed used for 2
> > different things: a new stack pointer for a user thread, or a function to be
> > executed by a kernel thread. Rather than "arg1", what would you think of
> > something like "sp_or_fn", or "usp_or_fn"?
> I would recommend exactly "arg" since it can be used for multiple purposes
> and if the formal could ever be used for a third purpose we don't want to
> go through another re-naming patch to change it from sp_or_fn or
> usp_or_fn.
>
> If that's done, then the comment above the function could define what arg
> can represent.
Do others concur with this idea? Personally, I feel the code will be 
more readable/maintainable if the naming of args/variables/etc reflects 
what they are actually used for.

(Case in point: on IA64, copy_thread() adds the kernel thread arg to the 
user stack pointer. The kernel thread arg is always 0 when forking a 
user process, so this "works", but it's certainly not what the author 
intended. Good names make it harder to write buggy code!)

For readability, using the same arg for 2 different purposes is a bad 
practice (though it might be good for keeping the object code small). I 
hate to think that "arg" might be co-opted for another purpose again.
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