/dev/random entropy on readonly file system

My hostapd works very well on my raspberry Pi3. But if i put root filesystem in readonly, it does not work. I have put some folders like /var/run and /tmp in a ramdisk filesystem.

I have seen something in log files when the root filesystem is readonly:

random: Cannot read from /dev/random: Resource temporarily unavailable
random: Got 0/20 bytes from /dev/random
random: Only 0/20 bytes of strong random data available from /dev/random
random: Not enough entropy pool available for secure operations

So i think something is wrong with /dev/random when the root filesystem is in readonly mode.

What should i do to fix that ?