linux-kernel - Re: [PATCH V4 6/9] virtio-ccw: implement synchronize

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Date:   Thu, 12 May 2022 11:29:38 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Vineeth Vijayan <vneethv@...ux.ibm.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        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>,
        eperezma <eperezma@...hat.com>, Cindy Lu <lulu@...hat.com>,
        Stefano Garzarella <sgarzare@...hat.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        Peter Oberparleiter <oberpar@...ux.ibm.com>
Subject: Re: [PATCH V4 6/9] virtio-ccw: implement synchronize_cbs()

On Wed, May 11, 2022 at 10:52 PM Vineeth Vijayan <vneethv@...ux.ibm.com> wrote:
>
> On Wed, May 11, 2022 at 05:28:11PM +0800, Jason Wang wrote:
> > On Wed, May 11, 2022 at 5:13 PM Cornelia Huck <cohuck@...hat.com> wrote:
> > >
> > > On Wed, May 11 2022, Jason Wang <jasowang@...hat.com> wrote:
> > >
> > > > On Wed, May 11, 2022 at 4:17 PM Cornelia Huck <cohuck@...hat.com> wrote:
> > > >>
> > > >> On Wed, May 11 2022, Jason Wang <jasowang@...hat.com> wrote:
> > > >>
> > > >> > 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:
> > > >> >> > @@ -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().
> > > >>
> > > >> io_subchannel_quiesce() should disable interrupts, though? Otherwise, it
> > > >> would be a bug.
> > > >
> > > > Right, it was protected by a spin_lock_irq(), but I can see other
> > > > cdev->handler() in e.g device_fsm.c, the irq status is not obvious, do
> > > > they have the same assumption which IRQ is disabled?
> > >
> > > Yes, that should be the case for any invocations via the fsm as well.
> > >
> >
> > Ok.
> >
> > > It's been some time since I've worked on that part of the code, though,
> > > so let's cc: the s390 cio maintainers so that they can speak up if I'm
> > > wrong.
> >
> > Ok, I will do that.
> >
> > Thanks
> >
> > >
> Thank you Corny to looking in to this. I agree, the cdev->handler is
> called with lock held. And as you mentioned, in the fsm these handler
> invocations are done with IRQ disabled, which will otherwise end up in a
> deadlock.
> thanks.
>

Thanks a lot for the confirmation, I will use
spin_lock()/spin_unlock() in the next version.