Linux Virtualization - Jul 2019 - INFO: rcu detected stall in vhost_worker

On Fri, Jul 26, 2019 at 8:26 AM syzbot
<syzbot+36e93b425cd6eb54fcc1 at syzkaller.appspotmail.com>
wrote:
>
> syzbot has bisected this bug to:
>
> commit 0ecfebd2b52404ae0c54a878c872bb93363ada36
> Author: Linus Torvalds <torvalds at linux-foundation.org>
> Date:   Sun Jul 7 22:41:56 2019 +0000
>
>      Linux 5.2
That seems very unlikely. That commit literally just changes the
EXTRAVERSION part of the version string.

So even if something actually depended on the version number, even
that wouldn't have triggered any semantic change.

              Linus

Fri, 26 Jul 2019 08:26:01 -0700 (PDT)
> syzbot has bisected this bug to:
> 
> commit 0ecfebd2b52404ae0c54a878c872bb93363ada36
> Author: Linus Torvalds <torvalds at linux-foundation.org>
> Date:   Sun Jul 7 22:41:56 2019 +0000
> 
>      Linux 5.2
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=118810bfa00000
> start commit:   13bf6d6a Add linux-next specific files for 20190725
> git tree:       linux-next
> kernel config:  https://syzkaller.appspot.com/x/.config?x=8ae987d803395886
> dashboard link:
https://syzkaller.appspot.com/bug?extid=36e93b425cd6eb54fcc1
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15112f3fa00000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=131ab578600000
> 
> Reported-by: syzbot+36e93b425cd6eb54fcc1 at syzkaller.appspotmail.com
> Fixes: 0ecfebd2b524 ("Linux 5.2")
> 
> For information about bisection process see:
https://goo.gl/tpsmEJ#bisection
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -787,7 +787,6 @@ static void vhost_setup_uaddr(struct vho
 			      size_t size, bool write)
 {
 	struct vhost_uaddr *addr = &vq->uaddrs[index];
-	spin_lock(&vq->mmu_lock);
 
 	addr->uaddr = uaddr;
 	addr->size = size;
@@ -797,7 +796,10 @@ static void vhost_setup_uaddr(struct vho
 static void vhost_setup_vq_uaddr(struct vhost_virtqueue *vq)
 {
 	spin_lock(&vq->mmu_lock);
-
+	/*
+	 * deadlock if managing to take mmu_lock again while
+	 * setting up uaddr
+	 */
 	vhost_setup_uaddr(vq, VHOST_ADDR_DESC,
 			  (unsigned long)vq->desc,
 			  vhost_get_desc_size(vq, vq->num),
--

On Sat, Jul 27, 2019 at 04:23:23PM +0800, Hillf Danton
wrote:
> 
> Fri, 26 Jul 2019 08:26:01 -0700 (PDT)
> > syzbot has bisected this bug to:
> > 
> > commit 0ecfebd2b52404ae0c54a878c872bb93363ada36
> > Author: Linus Torvalds <torvalds at linux-foundation.org>
> > Date:   Sun Jul 7 22:41:56 2019 +0000
> > 
> >      Linux 5.2
> > 
> > bisection log: 
https://syzkaller.appspot.com/x/bisect.txt?x=118810bfa00000
> > start commit:   13bf6d6a Add linux-next specific files for 20190725
> > git tree:       linux-next
> > kernel config: 
https://syzkaller.appspot.com/x/.config?x=8ae987d803395886
> > dashboard link:
https://syzkaller.appspot.com/bug?extid=36e93b425cd6eb54fcc1
> > syz repro:     
https://syzkaller.appspot.com/x/repro.syz?x=15112f3fa00000
> > C reproducer:  
https://syzkaller.appspot.com/x/repro.c?x=131ab578600000
> > 
> > Reported-by: syzbot+36e93b425cd6eb54fcc1 at syzkaller.appspotmail.com
> > Fixes: 0ecfebd2b524 ("Linux 5.2")
> > 
> > For information about bisection process see:
https://goo.gl/tpsmEJ#bisection
> 
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -787,7 +787,6 @@ static void vhost_setup_uaddr(struct vho
> 			      size_t size, bool write)
> {
> 	struct vhost_uaddr *addr = &vq->uaddrs[index];
> -	spin_lock(&vq->mmu_lock);
> 
> 	addr->uaddr = uaddr;
> 	addr->size = size;
> @@ -797,7 +796,10 @@ static void vhost_setup_uaddr(struct vho
> static void vhost_setup_vq_uaddr(struct vhost_virtqueue *vq)
> {
> 	spin_lock(&vq->mmu_lock);
> -
> +	/*
> +	 * deadlock if managing to take mmu_lock again while
> +	 * setting up uaddr
> +	 */
> 	vhost_setup_uaddr(vq, VHOST_ADDR_DESC,
> 			  (unsigned long)vq->desc,
> 			  vhost_get_desc_size(vq, vq->num),
> --
Thanks!
I reverted this whole commit.

-- 
MST