When is it safe to remove USB stick with kernel?

[donguss]

For example, our kernel resides on the USB stick and we plug it in every time we boot our machine. Let's suppose it's UEFI setup and the kernel is efi stub.
On which stage of boot process is it safe to remove the stick wothout fearing of kernel data corruption?

[bunder]

I would say somewhere between (the "pivot" between initramfs and openrc/systemd starting) and the root and localmount services starting.
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[szatox]

With a bootloader it's easy, as soon as kernel and initramfs are loaded (and kernel takes over). So, when you see the first output line printed by kernel, it's already safe to remove boot disk.
Not sure how efi boot works. I suppose it also starts with loading kernel to memory and THEN hands control over, so the same rules apply. Kernel takes over -> you can remove your stick.