I'm not really sure when it gets run.  Heres the time-line I'm generally
familiar with.  The parties involved are kernel, udev, initramfs-tools
and open-iscsi's local-top hook (/usr/share/initramfs-tools/scripts
/local-top/iscsi).

a. initramfs is built
b. kernel boot with set of command line parameters
c. initramfs calls 'local-top' scripts (see initramfs-tools(8))
d. open-iscsi reads kernel parameters, and if parameters are there,
   i. starts networking
   ii. sets up the iscsi target device
e. something (I'm pretty sure its udev) puts an entry into /dev/by-path for 
this new device that the kernel knows about. [1]
f. initramfs code takes over again, and calls 'mountroot' which does 
'wait-for-root'.  wait-for-root blocks for up to 30 seconds for the provided 
ROOT= device to appear.

We do block for up to 30 seconds, and also there is 'wait_for_udev', so
there should not be any race involved in using this.

So, if the path is consistent/reliable, then this seem safe to me.

--
[1] 
https://wiki.archlinux.org/index.php/Persistent_block_device_naming#by-id_and_by-path

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https://bugs.launchpad.net/bugs/1075313

Title:
  no reliable way to boot from iscsi root

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