linux-kernel - Re: [PATCH] usb: hub: Minor refactoring in usb_hub

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Date:   Mon, 23 Sep 2019 10:58:50 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Greg KH <gregkh@...uxfoundation.org>
cc:     Austin Kim <austindh.kim@...il.com>,
        <linux-kernel@...r.kernel.org>, <linux-usb@...r.kernel.org>,
        <mathias.nyman@...ux.intel.com>, <Thinh.Nguyen@...opsys.com>,
        <nsaenzjulienne@...e.de>, <jflat@...omium.org>, <malat@...ian.org>,
        <dianders@...omium.org>
Subject: Re: [PATCH] usb: hub: Minor refactoring in usb_hub_init()

On Mon, 23 Sep 2019, Greg KH wrote:

> On Mon, Sep 23, 2019 at 03:19:21PM +0900, Austin Kim wrote:
> > Normally when creation of workqueue fails, exception handling takes place
> > after the call to alloc_workqueue() is made.
> > 
> > But looking into usb_hub_init() function, 'return 0' statement is executed,
> > when alloc_workqueue() returns valid workqueue pointer.
> >        if (hub_wq)
> >                return 0;
> > 
> > This might make other Linux driver developers get confused 
> > because they could deduce that this is exceptional handling routine.
> > 
> > So perform minor refactoring by adding NULL pointer dereference check
> > routine right after the call to alloc_workqueue() is made.
> > 
> > Signed-off-by: Austin Kim <austindh.kim@...il.com>
> > ---
> >  drivers/usb/core/hub.c | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/usb/core/hub.c b/drivers/usb/core/hub.c
> > index e8ebacc..0ddbfe6 100644
> > --- a/drivers/usb/core/hub.c
> > +++ b/drivers/usb/core/hub.c
> > @@ -5530,9 +5530,12 @@ int usb_hub_init(void)
> >  	 * over to the companion full-speed controller.
> >  	 */
> >  	hub_wq = alloc_workqueue("usb_hub_wq", WQ_FREEZABLE, 0);
> > -	if (hub_wq)
> > -		return 0;
> > +	if (unlikely(!hub_wq))
> 
> Only ever use likely/unlikely if you can measure the difference without
> it.  Otherwise the compiler and cpu will almost always do this better
> than you.
> 
> So please remove this.
> 
> > +		goto err_workqueue;
> > +
> > +	return 0;
> >  
> > +err_workqueue:
> >  	/* Fall through if kernel_thread failed */
> 
> This comment is now incorrect.
> 
> But really, there is nothing wrong with the original code here.  It
> works properly, and while it is not identical to normal code "style"
> here, there's nothing wrong with it that I can see.

Indeed.  In fact, I suspect that this change would make the code less 
understandable, because the reader would wonder why anybody would go to 
the trouble of jumping over a return statement.  After all, this:

	if (!test)
		jump error;
	return 0;
 error:

just looks like a strange and inefficient way of writing:

	if (test)
		return 0;

Anyone reading it would wonder what the original author was thinking.

If you really want to fix up this subroutine, you could change the two 
"return -1" statements.  They should return an appropriate error code, 
not just -1.

Alan Stern