How do some real time vendors license Linux when it is already open source?
I have had a hard time finding licensing models for companies/distributions like Wind River Linux and RedHawk Linux.
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Typically vendors such as RedHat, WindRiver, SUSE Enterprise, etc. license support for Linux. The packages they provide will be licensed under open source license such as MIT, GPLv2, BSD, etc. just like a typical Linux distribution. For licenses that require source distribution, e.g. GPLv2, these vendors certainly will comply and give you the full source.
What is entailed in the "support" that vendors supply? Security updates, newer versions of the packages, and debugging the software they provide. They sometimes might even help you debug applications you write on top of the software they support or debug their software running on your custom hardware. If you've ever tried to debug a kernel oops or a user-space application that you didn't write, you know it's not always the easiest to find the problem in somebody else's code. This is where the Linux vendors excel.
They're not just selling you the free software, they make the individual software components work together, debug your setup, and provide updates for both security and new features.
Edit: Note that Linux can become preemptive with the preempt-rt patch set and a kernel configuration option, so I geared my answer towards general commercial Linux. It is worth noting that not all commercial Linux vendors may support the preempt-rt flavor of Linux.