Stop feed when spindle rpm lower

Hi,
I looking for something can stop feed when spindle rpm is lower what is setup in G code S300 for example -15% and start feed when rpm back to S300.
 

This is normally what motion.spindle-at-speed is used for.
LinuxCNC checks motion.spindle-at-speed  after the
spindle speed has changed in the Gcode and pauses
the first cutting move after the spindle speed change until
motion.spindle-at-speed is true. 

If you have a way to read the spindle speed, usually the
"near" hal component is used to generate the spindle-at-speed
signal. This can also be done if the spindle drive has a signal to report
that is is at speed.

Note that this will not work if you want to stop or alter the feed rate
if the spindle slows during operation, as motion.spindle-at-speed is ignored
after the first cutting move that follows a spindle speed change.

If you need to alter or stop the feed if the spindle speed drops during operation,
you would need to use adaptive feed (motion.adaptive-speed) and some hal math
or logic.