linux-kernel - Re: [PATCH] sysfs: use a separate locking class for open files depending on mmap

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Date:	Wed, 20 Nov 2013 01:30:39 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Dave Jones <davej@...hat.com>, Al Viro <viro@...IV.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sysfs: use a separate locking class for open files
 depending on mmap

On Sun, Nov 17, 2013 at 08:45:23PM -0800, Greg Kroah-Hartman wrote:
> On Sun, Nov 17, 2013 at 12:29:37PM +0900, Tejun Heo wrote:
> > On Sat, Nov 16, 2013 at 10:21:19PM -0500, Dave Jones wrote:
> > > On Sun, Nov 17, 2013 at 11:17:36AM +0900, Tejun Heo wrote:
> > > 
> > >  
> > >  > +	if (has_mmap)
> > >  > +		mutex_init(&of->mutex);
> > >  > +	else
> > >  > +		mutex_init(&of->mutex);
> > > 
> > > ummm...
> > 
> > Supposed to look that way.  It'll give two separate static lock class
> > keys to of->mutex.  Yeah, looks weird.  Any better ideas?
> 
> Doesn't gcc optimize that away to just one lock class anyway?

Well, it basically becomes

	if (has_mmap) {
		static struct lock_class_key key;
		__mutex_init(blah, &key);
	} else {
		static struct lock_class_key key;
		__mutex_init(blah, &key);
	}

So, the compiler isn't allowed to merge the two keys and we actually
use constructs like the above from a few places.  Yeah, it is weird
but does work.

Thanks.

-- 
tejun
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