linux-kernel - Re: [PATCH v2 4/4] usb: hub: rename khubd to hub

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Date:	Thu, 18 Sep 2014 10:24:23 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Tejun Heo <tj@...nel.org>
cc:	Petr Mladek <pmladek@...e.cz>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Joe Lawrence <joe.lawrence@...atus.com>,
	Jiri Kosina <jkosina@...e.cz>, <linux-usb@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 4/4] usb: hub: rename khubd to hub_wq in documentation
 and comments

On Thu, 18 Sep 2014, Tejun Heo wrote:

> Hello, Alan, Petr.
> 
> On Wed, Sep 17, 2014 at 01:36:26PM -0400, Alan Stern wrote:
> > > -	/* If khubd ever becomes multithreaded, this will need a lock */
> > > +	/* If hub_wq ever becomes multithreaded, this will need a lock */
> > >  	if (udev->wusb) {
> > >  		devnum = udev->portnum + 1;
> > >  		BUG_ON(test_bit(devnum, bus->devmap.devicemap));
> > 
> > You probably didn't notice when changing this comment.  But in fact,
> > workqueues _are_ multithreaded.  Therefore you need to add a lock to 
> > this routine.
> 
> Haven't read the code but if this function is called from a single
> work_struct, workqueue guarantees that there's only single thread of
> execution at any given time.  A work item is never executed
> concurrently no matter what.

This routine can be called from multiple work_structs, because a USB 
bus can have multiple hubs.

Alan Stern

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