Re: INFO: rcu detected stall in vhost_worker

Re: INFO: rcu detected stall in vhost_worker

[Linus Torvalds]

On Fri, Jul 26, 2019 at 8:26 AM syzbot
<syzbot+36e93b425cd6eb54fcc1@syzkaller.appspotmail.com> wrote:
>
> syzbot has bisected this bug to:
>
> commit 0ecfebd2b52404ae0c54a878c872bb93363ada36
> Author: Linus Torvalds <torvalds@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

Re: INFO: rcu detected stall in vhost_worker

[Hillf Danton]

Fri, 26 Jul 2019 08:26:01 -0700 (PDT)
> syzbot has bisected this bug to:
> 
> commit 0ecfebd2b52404ae0c54a878c872bb93363ada36
> Author: Linus Torvalds <torvalds@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@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),
--

Re: INFO: rcu detected stall in vhost_worker

[Michael S. Tsirkin]

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@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@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