Since we upgraded from v2.6.3 to v2.6.4, we are experiencing some problems with our php-fpm backend

The problem is that the php-fpm process gets stuck in a "read" state (and not go back to "accept" state).
Eventually php's pm.max_children is reached and the entire server will be killed by ec2's health checker.

I've done some debugging with strace, but have not found a pattern, because the servers gets lots of traffic, but the problem only occurs every once in a while. (maybe once in a 1000 requests)

root@frontend:~# strace -p 39614
strace: Process 39614 attached
read(5, 

root@frontend:~# nsenter -t 39614 --net lsof -n -p 39614 | grep 5u
php-fpm 39614 www-data    5u  IPv6 287757      0t0     TCP 172.18.0.3:9000->172.18.0.4:44492 (ESTABLISHED)

172.18.0.3 = app_php_1.app_default
172.18.0.4 = app_caddy_1.app_default

Im afraid that #5367 might need to be revisited.
I've reverted back to v2.6.3 and the problem is gone.