Re: [PATCH 2/2] scsi: Await request purging

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Feb 02, 2024 at 03:47:55PM +0100, Hanna Czenczek wrote:
> scsi_device_for_each_req_async() currently does not provide any way to
> be awaited.  One of its callers is scsi_device_purge_requests(), which
> therefore currently does not guarantee that all requests are fully
> settled when it returns.
> 
> We want all requests to be settled, because scsi_device_purge_requests()
> is called through the unrealize path, including the one invoked by
> virtio_scsi_hotunplug() through qdev_simple_device_unplug_cb(), which
> most likely assumes that all SCSI requests are done then.
> 
> In fact, scsi_device_purge_requests() already contains a blk_drain(),
> but this will not fully await scsi_device_for_each_req_async(), only the
> I/O requests it potentially cancels (not the non-I/O requests).
> However, we can have scsi_device_for_each_req_async() increment the BB
> in-flight counter, and have scsi_device_for_each_req_async_bh()
> decrement it when it is done.  This way, the blk_drain() will fully
> await all SCSI requests to be purged.
> 
> This also removes the need for scsi_device_for_each_req_async_bh() to
> double-check the current context and potentially re-schedule itself,
> should it now differ from the BB's context: Changing a BB's AioContext
> with a root node is done through bdrv_try_change_aio_context(), which
> creates a drained section.  With this patch, we keep the BB in-flight
> counter elevated throughout, so we know the BB's context cannot change.
> 
> Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
> ---
>  hw/scsi/scsi-bus.c | 30 +++++++++++++++++++++---------
>  1 file changed, 21 insertions(+), 9 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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