linux-kernel - Re: [PATCH RFC] virtio: hint if callbacks surprisingly might sleep

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Message-ID: <20190131164146.285c69d1.cohuck@redhat.com>
Date:   Thu, 31 Jan 2019 16:41:46 +0100
From:   Cornelia Huck <cohuck@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Jason Wang <jasowang@...hat.com>,
        Halil Pasic <pasic@...ux.ibm.com>, linux-s390@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] virtio: hint if callbacks surprisingly might sleep

On Thu, 31 Jan 2019 10:27:53 -0500
"Michael S. Tsirkin" <mst@...hat.com> wrote:

> On Thu, Jan 31, 2019 at 01:53:14PM +0100, Cornelia Huck wrote:
> > A virtio transport is free to implement some of the callbacks in
> > virtio_config_ops in a matter that they cannot be called from
> > atomic context (e.g. virtio-ccw, which maps a lot of the callbacks
> > to channel I/O, which is an inherently asynchronous mechanism).
> > This can be very surprising for developers using the much more
> > common virtio-pci transport, just to find out that things break
> > when used on s390.
> > 
> > The documentation for virtio_config_ops now contains a comment
> > explaining this, but it makes sense to add a might_sleep() annotation
> > to various wrapper functions in the virtio core to avoid surprises
> > later.
> > 
> > Note that annotations are NOT added to two classes of calls:
> > - direct calls from device drivers (all current callers should be
> >   fine, however)
> > - calls which clearly won't be made from atomic context (such as
> >   those ultimately coming in via the driver core)
> > 
> > Signed-off-by: Cornelia Huck <cohuck@...hat.com>  
> 
> 
> Makes sense to me. I don't think we should push our luck in
> this release though, better defer until the merge window.

Nod, that's definitely something for the next release.