Date:   Wed, 11 May 2022 10:41:56 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     virtualization <virtualization@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Marc Zyngier <maz@...nel.org>,
        Halil Pasic <pasic@...ux.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        eperezma <eperezma@...hat.com>, Cindy Lu <lulu@...hat.com>,
        Stefano Garzarella <sgarzare@...hat.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Subject: Re: [PATCH V4 6/9] virtio-ccw: implement synchronize_cbs()

On Tue, May 10, 2022 at 7:28 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Sat, May 07, 2022 at 03:19:51PM +0800, Jason Wang wrote:
> > This patch tries to implement the synchronize_cbs() for ccw. For the
> > vring_interrupt() that is called via virtio_airq_handler(), the
> > synchronization is simply done via the airq_info's lock. For the
> > vring_interrupt() that is called via virtio_ccw_int_handler(), a per
> > device spinlock for irq is introduced ans used in the synchronization
> > method.
> >
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: "Paul E. McKenney" <paulmck@...nel.org>
> > Cc: Marc Zyngier <maz@...nel.org>
> > Cc: Halil Pasic <pasic@...ux.ibm.com>
> > Cc: Cornelia Huck <cohuck@...hat.com>
> > Signed-off-by: Jason Wang <jasowang@...hat.com>
> > ---
> >  drivers/s390/virtio/virtio_ccw.c | 27 +++++++++++++++++++++++++++
> >  1 file changed, 27 insertions(+)
> >
> > diff --git a/drivers/s390/virtio/virtio_ccw.c b/drivers/s390/virtio/virtio_ccw.c
> > index d35e7a3f7067..001e1f0e6037 100644
> > --- a/drivers/s390/virtio/virtio_ccw.c
> > +++ b/drivers/s390/virtio/virtio_ccw.c
> > @@ -62,6 +62,7 @@ struct virtio_ccw_device {
> >       unsigned int revision; /* Transport revision */
> >       wait_queue_head_t wait_q;
> >       spinlock_t lock;
> > +     rwlock_t irq_lock;
> >       struct mutex io_lock; /* Serializes I/O requests */
> >       struct list_head virtqueues;
> >       bool is_thinint;
> > @@ -984,6 +985,27 @@ static const char *virtio_ccw_bus_name(struct virtio_device *vdev)
> >       return dev_name(&vcdev->cdev->dev);
> >  }
> >
> > +static void virtio_ccw_synchronize_cbs(struct virtio_device *vdev)
> > +{
> > +     struct virtio_ccw_device *vcdev = to_vc_device(vdev);
> > +     struct airq_info *info = vcdev->airq_info;
> > +
> > +     if (info) {
> > +             /*
> > +              * Synchronize with the vring_interrupt() with airq indicator
> > +              */
> > +             write_lock(&info->lock);
> > +             write_unlock(&info->lock);
> > +     } else {
> > +             /*
> > +              * Synchronize with the vring_interrupt() called by
> > +              * virtio_ccw_int_handler().
> > +              */
> > +             write_lock(&vcdev->irq_lock);
> > +             write_unlock(&vcdev->irq_lock);
> > +     }
> > +}
> > +
> >  static const struct virtio_config_ops virtio_ccw_config_ops = {
> >       .get_features = virtio_ccw_get_features,
> >       .finalize_features = virtio_ccw_finalize_features,
> > @@ -995,6 +1017,7 @@ static const struct virtio_config_ops virtio_ccw_config_ops = {
> >       .find_vqs = virtio_ccw_find_vqs,
> >       .del_vqs = virtio_ccw_del_vqs,
> >       .bus_name = virtio_ccw_bus_name,
> > +     .synchronize_cbs = virtio_ccw_synchronize_cbs,
> >  };
> >
> >
> > @@ -1079,6 +1102,7 @@ static void virtio_ccw_int_handler(struct ccw_device *cdev,
> >  {
> >       __u32 activity = intparm & VIRTIO_CCW_INTPARM_MASK;
> >       struct virtio_ccw_device *vcdev = dev_get_drvdata(&cdev->dev);
> > +     unsigned long flags;
> >       int i;
> >       struct virtqueue *vq;
> >
> > @@ -1106,6 +1130,7 @@ static void virtio_ccw_int_handler(struct ccw_device *cdev,
> >                       vcdev->err = -EIO;
> >       }
> >       virtio_ccw_check_activity(vcdev, activity);
> > +     read_lock_irqsave(&vcdev->irq_lock, flags);
> >       for_each_set_bit(i, indicators(vcdev),
> >                        sizeof(*indicators(vcdev)) * BITS_PER_BYTE) {
> >               /* The bit clear must happen before the vring kick. */
>
> Cornelia sent a lockdep trace on this.
>
> Basically I think this gets the irqsave/restore logic wrong.
> It attempts to disable irqs in the handler (which is an interrupt
> anyway).

The reason I use irqsave/restore is that it can be called from process
context (if I was not wrong), e.g from io_subchannel_quiesce().

> And it does not disable irqs in the synchronize_cbs.
>
> As a result in interrupt might try to take a read lock while
> .synchronize_cbs has the writer lock, resulting in a deadlock.
>
> I think you want regular read_lock + write_lock_irq.

Yes.

Thanks

>
>
> > @@ -1114,6 +1139,7 @@ static void virtio_ccw_int_handler(struct ccw_device *cdev,
> >               vq = virtio_ccw_vq_by_ind(vcdev, i);
> >               vring_interrupt(0, vq);
> >       }
> > +     read_unlock_irqrestore(&vcdev->irq_lock, flags);
> >       if (test_bit(0, indicators2(vcdev))) {
> >               virtio_config_changed(&vcdev->vdev);
> >               clear_bit(0, indicators2(vcdev));
> > @@ -1284,6 +1310,7 @@ static int virtio_ccw_online(struct ccw_device *cdev)
> >       init_waitqueue_head(&vcdev->wait_q);
> >       INIT_LIST_HEAD(&vcdev->virtqueues);
> >       spin_lock_init(&vcdev->lock);
> > +     rwlock_init(&vcdev->irq_lock);
> >       mutex_init(&vcdev->io_lock);
> >
> >       spin_lock_irqsave(get_ccwdev_lock(cdev), flags);
> > --
> > 2.25.1
>

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