linux-kernel - Re: [DRBD-user] [PATCH] xen-blkback: Switch to closed state after releasing the backing device

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Date:   Fri, 7 Sep 2018 14:03:37 +0200
From:   Lars Ellenberg <lars.ellenberg@...bit.com>
To:     Valentin Vidic <Valentin.Vidic@...Net.hr>
Cc:     Roger Pau Monné <roger.pau@...rix.com>,
        Jens Axboe <axboe@...nel.dk>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        linux-block@...r.kernel.org, xen-devel@...ts.xenproject.org,
        drbd-user@...ts.linbit.com
Subject: Re: [DRBD-user] [PATCH] xen-blkback: Switch to closed state after
 releasing the backing device

On Wed, Sep 05, 2018 at 06:27:56PM +0200, Valentin Vidic wrote:
> On Wed, Sep 05, 2018 at 12:36:49PM +0200, Roger Pau Monné wrote:
> > On Wed, Aug 29, 2018 at 08:52:14AM +0200, Valentin Vidic wrote:
> > > Switching to closed state earlier can cause the block-drbd
> > > script to fail with 'Device is held open by someone':
> > > 
> > > root: /etc/xen/scripts/block-drbd: remove XENBUS_PATH=backend/vbd/6/51712
> > > kernel: [ 2222.278235] block drbd6: State change failed: Device is held open by someone
> > > kernel: [ 2222.278304] block drbd6:   state = { cs:Connected ro:Primary/Secondary ds:UpToDate/UpToDate r----- }
> > > kernel: [ 2222.278340] block drbd6:  wanted = { cs:Connected ro:Secondary/Secondary ds:UpToDate/UpToDate r----- }
> > > root: /etc/xen/scripts/block-drbd: Writing backend/vbd/6/51712/hotplug-error /etc/xen/scripts/block-drbd failed; error detected. backend/vbd/6/51712/hotplug-status error to xenstore.
> > > root: /etc/xen/scripts/block-drbd: /etc/xen/scripts/block-drbd failed; error detected.

Very frequently it is *NOT* the "original user", that "still" holds it
open, but udev, or something triggered-by-udev.

So double-checking the udev rules,
or the "lvm global_filter" settings may help.
You could instrument DRBD to log current->{pid,comm} on open and close,
so you can better detect who the "someone" is in the message above.

Adding a small retry loop in the script may help as well.

As a side note, 
with DRBD 9 (which still moves too fast to be "in tree")
we have the "auto-promote" feature, where no explicit promotion/demotion
is necessary anymore, but DRBD (tries to) promote itself with the
first "RW" opener, and demote itself once the last opener is gone.


Hth,
    Lars