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Date:   Mon, 13 Jun 2022 02:58:54 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Marcel Holtmann <marcel@holtmann.org>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Johan Hedberg <johan.hedberg@gmail.com>,
        Luiz Augusto von Dentz <luiz.dentz@gmail.com>,
        linux-bluetooth@vger.kernel.org,
        virtualization@lists.linux-foundation.org
Subject: Re: [PATCH] Bluetooth: virtio_bt: fix device removal
Message-ID: <20220613025539-mutt-send-email-mst@kernel.org>
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On Fri, Jan 14, 2022 at 03:12:47PM -0500, Michael S. Tsirkin wrote:
> On Thu, Dec 16, 2021 at 08:58:31PM +0100, Marcel Holtmann wrote:
> > Hi Michael,
> > 
> > >>>>> Device removal is clearly out of virtio spec: it attempts to remove
> > >>>>> unused buffers from a VQ before invoking device reset. To fix, make
> > >>>>> open/close NOPs and do all cleanup/setup in probe/remove.
> > >>>> 
> > >>>> so the virtbt_{open,close} as NOP is not really what a driver is suppose
> > >>>> to be doing. These are transport enable/disable callbacks from the BT
> > >>>> Core towards the driver. It maps to a device being enabled/disabled by
> > >>>> something like bluetoothd for example. So if disabled, I expect that no
> > >>>> resources/queues are in use.
> > >>>> 
> > >>>> Maybe I misunderstand the virtio spec in that regard, but I would like
> > >>>> to keep this fundamental concept of a Bluetooth driver. It does work
> > >>>> with all other transports like USB, SDIO, UART etc.
> > >>>> 
> > >>>>> The cost here is a single skb wasted on an unused bt device - which
> > >>>>> seems modest.
> > >>>> 
> > >>>> There should be no buffer used if the device is powered off. We also don’t
> > >>>> have any USB URBs in-flight if the transport is not active.
> > >>>> 
> > >>>>> NB: with this fix in place driver still suffers from a race condition if
> > >>>>> an interrupt triggers while device is being reset. Work on a fix for
> > >>>>> that issue is in progress.
> > >>>> 
> > >>>> In the virtbt_close() callback we should deactivate all interrupts.
> > >>>> 
> > >>>> Regards
> > >>>> 
> > >>>> Marcel
> > >>> 
> > >>> So Marcel, do I read it right that you are working on a fix
> > >>> and I can drop this patch for now?
> > >> 
> > >> ping
> > > 
> > > 
> > > If I don't hear otherwise I'll queue my version - it might not
> > > be ideal but it at least does not violate the spec.
> > > We can work on not allocating/freeing buffers later
> > > as appropriate.
> > 
> > I have a patch, but it is not fully tested yet.
> > 
> > Regards
> > 
> > Marcel
> 
> ping
> 
> it's been a month ...
> 
> I'm working on cleaning up module/device removal in virtio and bt
> is kind of sticking out.

I am inclined to make this driver depend on BROKEN for now.
Any objections?


> -- 
> MST