> From: Michael S. Tsirkin <mst@xxxxxxxxxx>
> Sent: Tuesday, May 27, 2025 1:45 AM
>
> On Mon, May 26, 2025 at 02:40:33PM +0000, Parav Pandit wrote:
> > When the PCI device is surprise-removed, requests may not complete on
> > the device as the VQ is marked as broken. As a result, disk deletion
> > hangs.
> >
> > Fix it by aborting the requests when the VQ is broken.
> >
> > With this fix now fio completes swiftly.
> > An alternative of IO timeout has been considered, however when the
> > driver knows about unresponsive block device, swiftly clearing them
> > enables users and upper layers to react quickly.
> >
> > Verified with multiple device unplug iterations with pending requests
> > in virtio used ring and some pending with the device.
> >
> > Fixes: 43bb40c5b926 ("virtio_pci: Support surprise removal of virtio
> > pci device")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Reported-by: Li RongQing <lirongqing@xxxxxxxxx>
> > Closes:
> > https://lore.kernel.org/virtualization/c45dd68698cd47238c55fb73ca9b474
> > 1@xxxxxxxxx/
> > Reviewed-by: Max Gurtovoy <mgurtovoy@xxxxxxxxxx>
> > Reviewed-by: Israel Rukshin <israelr@xxxxxxxxxx>
> > Signed-off-by: Parav Pandit <parav@xxxxxxxxxx>
>
>
> Thanks!
> I like the patch. Yes something to improve:
>
> > ---
> > v1->v2:
> > - Addressed comments from Stephan
> > - fixed spelling to 'waiting'
> > - Addressed comments from Michael
> > - Dropped checking broken vq from queue_rq() and queue_rqs()
> > because it is checked in lower layer routines in virtio core
> >
> > v0->v1:
> > - Fixed comments from Stefan to rename a cleanup function
> > - Improved logic for handling any outstanding requests
> > in bio layer
> > - improved cancel callback to sync with ongoing done()
> > ---
> > drivers/block/virtio_blk.c | 83
> > ++++++++++++++++++++++++++++++++++++++
> > 1 file changed, 83 insertions(+)
> >
> > diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> > index 7cffea01d868..12f31e6c00cb 100644
> > --- a/drivers/block/virtio_blk.c
> > +++ b/drivers/block/virtio_blk.c
> > @@ -1554,6 +1554,87 @@ static int virtblk_probe(struct virtio_device
> *vdev)
> > return err;
> > }
> >
> > +static bool virtblk_request_cancel(struct request *rq, void *data)
>
>
> > +{
> > + struct virtblk_req *vbr = blk_mq_rq_to_pdu(rq);
> > + struct virtio_blk *vblk = data;
> > + struct virtio_blk_vq *vq;
> > + unsigned long flags;
> > +
> > + vq = &vblk->vqs[rq->mq_hctx->queue_num];
> > +
> > + spin_lock_irqsave(&vq->lock, flags);
> > +
> > + vbr->in_hdr.status = VIRTIO_BLK_S_IOERR;
>
> My undertanding is that this is only safe to call when device is not accessing
> the vq anymore? Right? otherwise device can overwrite the status?
>
Even if the device is accessing the VQ (such as descriptors) and its memory, and if we free and unmap the request, it is a problem.
So the contract is, when the device has reported that the transport is broken, after that it must not touch the VQ.
The in_hdr.status is updated either here or in the done() handler.
And both are protected by spin lock, so they don't step on each other.
> But I am not sure I understand what guarantees this.
> Is there an assumption here that if vq is broken and we are on remove path
> then device is already gone?
True, the assumption is that device must not touch VQ or memory pointed by VQ when it has reported that device is broken.
> It seems to hold but
> I'd prefer something that makes this guarantee at the API level.
>
Not sure how we can guarantee that from the device.
It's the assumption from the driver in following the pci spec.
>
>
> > + if (blk_mq_request_started(rq) && !blk_mq_request_completed(rq))
> > + blk_mq_complete_request(rq);
> > +
> > + spin_unlock_irqrestore(&vq->lock, flags);
> > + return true;
> > +}
> > +
> > +static void virtblk_broken_device_cleanup(struct virtio_blk *vblk) {
> > + struct request_queue *q = vblk->disk->queue;
> > +
> > + if (!virtqueue_is_broken(vblk->vqs[0].vq))
> > + return;
>
> can marking vqs broken be in progress such that 0 is already broken but
> another one is not, yet?
> if not pls add a comment explainging why.
>
> > +
> > + /* Start freezing the queue, so that new requests keeps waiting at
> the
> > + * door of bio_queue_enter(). We cannot fully freeze the queue
> because
> > + * freezed queue is an empty queue and there are pending requests,
> so
> > + * only start freezing it.
> > + */
> > + blk_freeze_queue_start(q);
> > +
> > + /* When quiescing completes, all ongoing dispatches have completed
> > + * and no new dispatch will happen towards the driver.
> > + * This ensures that later when cancel is attempted, then are not
> > + * getting processed by the queue_rq() or queue_rqs() handlers.
> > + */
> > + blk_mq_quiesce_queue(q);
> > +
> > + /*
> > + * Synchronize with any ongoing VQ callbacks, effectively quiescing
> > + * the device and preventing it from completing further requests
> > + * to the block layer. Any outstanding, incomplete requests will be
> > + * completed by virtblk_request_cancel().
>
>
> I think what you really mean is:
> finish processing in callbacks, that might have started before vqs were
> marked as broken.
>
Yes, I will remove the word "effectively quiescing the device", because that is not done by pure sw code here.
It is quiescing the async interrupt handler side.
How about a below rephrase:
/*
* Synchronize with any ongoing VQ callbacks which may be started before VQs are marked broken,
* preventing it from completing further requests to the block layer. Any outstanding,
* incomplete requests will be completed by virtblk_request_cancel().
*/
>
>
> > + */
> > + virtio_synchronize_cbs(vblk->vdev);
>
>
>
>
> > +
> > + /* At this point, no new requests can enter the queue_rq() and
> > + * completion routine will not complete any new requests either for
> the
> > + * broken vq. Hence, it is safe to cancel all requests which are
> > + * started.
> > + */
> > + blk_mq_tagset_busy_iter(&vblk->tag_set, virtblk_request_cancel,
> vblk);
> > + blk_mq_tagset_wait_completed_request(&vblk->tag_set);
> > +
> > + /* All pending requests are cleaned up. Time to resume so that disk
> > + * deletion can be smooth. Start the HW queues so that when queue
> is
> > + * unquiesced requests can again enter the driver.
> > + */
> > + blk_mq_start_stopped_hw_queues(q, true);
> > +
> > + /* Unquiescing will trigger dispatching any pending requests to the
> > + * driver which has crossed bio_queue_enter() to the driver.
> > + */
> > + blk_mq_unquiesce_queue(q);
> > +
> > + /* Wait for all pending dispatches to terminate which may have been
> > + * initiated after unquiescing.
> > + */
> > + blk_mq_freeze_queue_wait(q);
> > +
> > + /* Mark the disk dead so that once queue unfreeze, the requests
> > + * waiting at the door of bio_queue_enter() can be aborted right
> away.
> > + */
> > + blk_mark_disk_dead(vblk->disk);
> > +
> > + /* Unfreeze the queue so that any waiting requests will be aborted.
> */
> > + blk_mq_unfreeze_queue_nomemrestore(q);
> > +}
> > +
> > static void virtblk_remove(struct virtio_device *vdev) {
> > struct virtio_blk *vblk = vdev->priv; @@ -1561,6 +1642,8 @@ static
> > void virtblk_remove(struct virtio_device *vdev)
> > /* Make sure no work handler is accessing the device. */
> > flush_work(&vblk->config_work);
> >
> > + virtblk_broken_device_cleanup(vblk);
> > +
> > del_gendisk(vblk->disk);
> > blk_mq_free_tag_set(&vblk->tag_set);
> >
> > --
> > 2.34.1
