linux-kernel - Re: [RFC][PATCH 2/7] kref: Add kref

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Date:   Tue, 15 Nov 2016 09:03:14 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     keescook@...omium.org, will.deacon@....com,
        elena.reshetova@...el.com, arnd@...db.de, tglx@...utronix.de,
        mingo@...nel.org, hpa@...or.com, dave@...gbits.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 2/7] kref: Add kref_read()

On Tue, Nov 15, 2016 at 08:33:22AM +0100, Greg KH wrote:
> On Mon, Nov 14, 2016 at 06:39:48PM +0100, Peter Zijlstra wrote:

> > --- a/drivers/block/drbd/drbd_req.c
> > +++ b/drivers/block/drbd/drbd_req.c
> > @@ -520,7 +520,7 @@ static void mod_rq_state(struct drbd_req
> >  		/* Completion does it's own kref_put.  If we are going to
> >  		 * kref_sub below, we need req to be still around then. */
> >  		int at_least = k_put + !!c_put;
> > -		int refcount = atomic_read(&req->kref.refcount);
> > +		int refcount = kref_read(&req->kref);
> >  		if (refcount < at_least)
> >  			drbd_err(device,
> >  				"mod_rq_state: Logic BUG: %x -> %x: refcount = %d, should be >= %d\n",
> 
> As proof of "things you should never do", here is one such example.
> 
> ugh.
> 
> 
> > --- a/drivers/block/virtio_blk.c
> > +++ b/drivers/block/virtio_blk.c
> > @@ -767,7 +767,7 @@ static void virtblk_remove(struct virtio
> >  	/* Stop all the virtqueues. */
> >  	vdev->config->reset(vdev);
> >  
> > -	refc = atomic_read(&disk_to_dev(vblk->disk)->kobj.kref.refcount);
> > +	refc = kref_read(&disk_to_dev(vblk->disk)->kobj.kref);
> >  	put_disk(vblk->disk);
> >  	vdev->config->del_vqs(vdev);
> >  	kfree(vblk->vqs);
> 
> And this too, ugh, that's a huge abuse and is probably totally wrong...
> 
> thanks again for digging through this crap.  I wonder if we need to name
> the kref reference variable "do_not_touch_this_ever" or some such thing
> to catch all of the people who try to be "too smart".

There's unimaginable bong hits involved in this stuff, in the end I
resorted to brute force and scripts to convert all this.