Re: [PATCH] Bluetooth: virtio_bt: fix device removal

From: Marcel Holtmann <marcel@holtmann.org>
To: "Michael S. Tsirkin" <mst@redhat.com>
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
Date: Thu, 25 Nov 2021 21:02:01 +0100	[thread overview]
Message-ID: <F52F65FE-6A07-486B-8E84-684ED85709E9@holtmann.org> (raw)
In-Reply-To: <20211125174200.133230-1-mst@redhat.com>

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