kernel-rt freezing at boot: "dracut: FATAL: No or empty root= argument"

Issue

kernel-rt freezing at boot with the following messages on console:

dracut Warning: dracut: FATAL: No or empty root= argument
dracut Warning: dracut: Refusing to continue
Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000100
CPU: 4 PID: 1 Comm: init Not tainted 3.10.58-rt62.58.el6rt.x86_64 #1
Hardware name: QCI QSSC-S4R/QSSC-S4R, BIOS QSSC-S4R.QCI.01.00.S013.032920111005 03/29/2011
ffffffff81a40540 ffff88045c8a3e28 ffffffff815786e3 ffff88045c8a3ea8
ffffffff81578439 ffff880400000010 ffff88045c8a3eb8 ffff88045c8a3e58
ffff88045c8a8000 ffff88045c8a2000 0000000000000100 0000000000000000
Call Trace:
    [<ffffffff815786e3>] dump_stack+0x19/0x1e
    [<ffffffff81578439>] panic+0xdc/0x20a
    [<ffffffff81054504>] do_exit+0x964/0xb70
    [<ffffffff8105476e>] do_group_exit+0x5e/0xe0
    [<ffffffff81054807>] SyS_exit_group+0x17/0x20
    [<ffffffff815844d9>] system_call_fastpath+0x16/0x1b