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




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