Access during Graceful Restart results in Internal Server Error(500)

[oss-aimoto]

It is recorded in the error log.

[Fri Sep 18 03:31:51.544799 2020] [mpm_prefork:notice] [pid 1] AH00171: Graceful restart requested, doing restart
[Fri Sep 18 03:31:51.564763 2020] [auth_mellon:error] [pid 3186] [client 127.0.0.1:55440] apr_global_mutex_lock() failed [22]: Invalid argument

Mutex freed by parent process during gracefull restart but child process still in process.
struct am_mod_cfg_rec can't be reused. I think the specified pool is incorrect.

Steps to reproduce

1. Log in to IdP (get a mellon session)
2. ab -n 10000 -c 20 -C mellon-cookie=[sessionid] https://example.co.jp:10001/test.html
3. kill -s USR1 [httpd pid]