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SubjectRe: [DRBD-user] [PATCH] xen-blkback: Switch to closed state after releasing the backing device
On Sat, Sep 08, 2018 at 09:34:32AM +0200, Valentin Vidic wrote:
> On Fri, Sep 07, 2018 at 07:14:59PM +0200, Valentin Vidic wrote:
> > In fact the first one is the original code path before I modified
> > blkback. The problem is it gets executed async from workqueue so
> > it might not always run before the call to drbdadm secondary.
>
> As the DRBD device gets released only when the last IO request
> has finished, I found a way to check and wait for this in the
> block-drbd script:

> --- block-drbd.orig 2018-09-08 09:07:23.499648515 +0200
> +++ block-drbd 2018-09-08 09:28:12.892193649 +0200
> @@ -230,6 +230,24 @@
> and so cannot be mounted ${m2}${when}."
> }
>
> +wait_for_inflight()
> +{
> + local dev="$1"
> + local inflight="/sys/block/${dev#/dev/}/inflight"
> + local rd wr
> +
> + if ! [ -f "$inflight" ]; then
> + return
> + fi
> +
> + while true; do
> + read rd wr < $inflight
> + if [ "$rd" = "0" -a "$wr" = "0" ]; then

If it is "idle" now, but still "open",
this will not sleep, and still fail the demotion below.

> + return
> + fi
> + sleep 1
> + done
> +}
>
> t=$(xenstore_read_default "$XENBUS_PATH/type" 'MISSING')
>
> @@ -285,6 +303,8 @@
> drbd_lrole="${drbd_role%%/*}"
> drbd_dev="$(drbdadm sh-dev $drbd_resource)"
>
> + wait_for_inflight $drbd_dev
> +
> if [ "$drbd_lrole" != 'Secondary' ]; then
> drbdadm secondary $drbd_resource

You try to help it by "waiting forever until it appears to be idle".
I suggest to at least limit the retries by iteration or time.
And also (or, instead; but you'd potentially get a number of
"scary messages" in the logs) add something like:
for i in 1 2 3 5 7 x; do
drbdadm secondary $drbd_resource && exit 0
if [ $i = x ]; then
# ... "appears to still be in use, maybe by" ...
fuser -v $drbd_dev
exit 1
# else ... "will retry in $i seconds" ...
fi
sleep $i
done
...

Or, well, yes, fix blkback to not "defer" the final close "too long",
if at all possible.

Lars

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