Re: [PATCH -next] virtio: balloon: fix missing unlock on error in fill_balloon()

[Rafael Aquini <aquini@redhat.com>]

On Mon, Nov 12, 2012 at 03:34:53PM -0800, Andrew Morton wrote:
> On Mon, 12 Nov 2012 21:50:40 +0800
> Wei Yongjun <weiyj.lk@gmail.com> wrote:
> 
> > From: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
> > 
> > Add the missing unlock before return from function fill_balloon()
> > in the error handling case.
> > 
> > Introduced by 9864a8(virtio_balloon: introduce migration primitives
> > to balloon pages)
> > 
> > dpatch engine is used to auto generate this patch.
> > (https://github.com/weiyj/dpatch)
> > 
> > Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
> > ---
> >  drivers/virtio/virtio_balloon.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> > index f70151b..72e8dcb 100644
> > --- a/drivers/virtio/virtio_balloon.c
> > +++ b/drivers/virtio/virtio_balloon.c
> > @@ -152,8 +152,10 @@ static void fill_balloon(struct virtio_balloon *vb, size_t num)
> >  	}
> >  
> >  	/* Didn't get any?  Oh well. */
> > -	if (vb->num_pfns == 0)
> > +	if (vb->num_pfns == 0) {
> > +		mutex_unlock(&vb->balloon_lock);
> >  		return;
> > +	}
> >  
> >  	tell_host(vb, vb->inflate_vq);
> >  	mutex_unlock(&vb->balloon_lock);
> 
> Well.  Why did this bug occur in the first place?  Because
> fill_balloon() has multiple return points and one of those was
> overlooked when adding new locking.
> 
> This (and the addition of memory leaks) is quite common.  It is part of
> the reason why so much kernel code uses goto's to avoid multiple return
> points.  A maintainability thing.
> 
> And we can fix this shortcoming in fill_balloon() without even needing a
> goto:
> 
> --- a/drivers/virtio/virtio_balloon.c~virtio_balloon-introduce-migration-primitives-to-balloon-pages-fix-fix
> +++ a/drivers/virtio/virtio_balloon.c
> @@ -151,13 +151,9 @@ static void fill_balloon(struct virtio_b
>  		totalram_pages--;
>  	}
>  
> -	/* Didn't get any?  Oh well. */
> -	if (vb->num_pfns == 0) {
> -		mutex_unlock(&vb->balloon_lock);
> -		return;
> -	}
> -
> -	tell_host(vb, vb->inflate_vq);
> +	/* Did we get any? */
> +	if (vb->num_pfns != 0)
> +		tell_host(vb, vb->inflate_vq);
>  	mutex_unlock(&vb->balloon_lock);
>  }
>

Your fix is far better, as it simplifies the code without loosing that test --
which only evaluates as true, if we fail to allocate a page right after getting
into the loop -- /me thinks driver folks place it there to not bother
tell_host() and the virt_queues if no pageframe is present at that pfns array.