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The race condition is noticed between disk_add() and disk attributes, on virtio-blk hotplug. 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. As pointed out by Christoph Hellwig in the specific fix [1], virtio-blk

The race condition is noticed between disk_add() and disk attributes, on virtio-blk hotplug. 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. As pointed out by Christoph Hellwig in the specific fix [1], virtio-blk

This is an attempt to fix the issue that some disks' sysfs attributes are not ready at the time its KOBJ_ADD event is sent. The symptom is during device hotplug, udev may fail to find certain attributes, such as serial or wwn, of the disk. As a result the /dev/disk/by-id entries are not created. The cause is device_add_disk emits the uevent before returning, and the callers have to create

This is an attempt to fix the issue that some disks' sysfs attributes are not ready at the time its KOBJ_ADD event is sent. The symptom is during device hotplug, udev may fail to find certain attributes, such as serial or wwn, of the disk. As a result the /dev/disk/by-id entries are not created. The cause is device_add_disk emits the uevent before returning, and the callers have to create