linux-kernel - Re: [PATCH] virtio-blk: Generate uevent after attribute available

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Date:	Tue, 28 Jun 2016 04:45:53 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Fam Zheng <famz@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	"Michael S. Tsirkin" <mst@...hat.com>,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] virtio-blk: Generate uevent after attribute available

On Tue, Jun 28, 2016 at 10:39:15AM +0800, Fam Zheng wrote:
> Userspace listens to the KOBJ_ADD uevent generated in add_disk. At that
> point we haven't created the serial attribute file, therefore depending
> on how fast udev reacts, the /dev/disk/by-id/ entry doesn't always get
> created.
> 
> This race condition can be easily reproduced by hot plugging a number of
> virtio-blk disks.
> 
> Also in systemd, there used to be a related workaround in udev rules
> called 'WAIT_FOR="serial"', but it is removed in later versions.
> 
> Now let's generate a KOBJ_CHANGE event after the attributes are ready.

The same race is present in other drivers as well, e.g. nvme.  Please
find a way to make this work properly without needing to hack every
driver to send events manually.