Any chance you can provide the actual crashing point, it seems like the debugging symbols is not loaded so its hard to tell what causes it exactly. (I realize based on your title it may be difficult, but it does help us debug it further)

Also if you have a script that seems to be the main reason for the cause, based off your logs, it would be nice to have included.

Can you also provide your configure command that was used to compile PHP?

Configured with:
./configure --prefix=/usr --build=x86_64-pc-linux-gnu --host=x86_64-pc-linux-gnu --mandir=/usr/share/man --infodir=/usr/share/info --datadir=/usr/share --sysconfdir=/etc --localstatedir=/var/lib --docdir=/usr/share/doc/php-5.6.31 --htmldir=/usr/share/doc/php-5.6.31/html --prefix=/usr/lib64/php5.6 --mandir=/usr/lib64/php5.6/man --infodir=/usr/lib64/php5.6/info --libdir=/usr/lib64/php5.6/lib --with-libdir=lib64 --localstatedir=/var --without-pear --disable-maintainer-zts --without-fpm-acl --disable-bcmath --with-bz2=/usr --disable-calendar --disable-gcov --enable-ctype --with-curl=/usr --enable-dom --without-enchant --disable-exif --enable-fileinfo --enable-filter --disable-ftp --with-gettext=/usr --without-gmp --enable-hash --without-mhash --with-iconv --enable-intl --disable-ipv6 --enable-json --without-kerberos --enable-libxml --with-libxml-dir=/usr --enable-mbstring --with-mcrypt=/usr --without-mssql --with-onig=/usr --with-openssl=/usr --with-openssl-dir=/usr --disable-pcntl --enable-phar --enable-pdo --enable-opcache --without-pgsql --enable-posix --without-pspell --without-recode --enable-simplexml --disable-shmop --without-snmp --disable-soap --disable-sockets --without-sqlite3 --without-sybase-ct --disable-sysvmsg --disable-sysvsem --disable-sysvshm --without-fpm-systemd --without-tidy --enable-tokenizer --disable-wddx --enable-xml --disable-xmlreader --disable-xmlwriter --without-xmlrpc --without-xsl --enable-zip --with-zlib=/usr --enable-debug --enable-dba --without-cdb --with-db4=/usr --disable-flatfile --without-gdbm --disable-inifile --without-qdbm --without-freetype-dir --without-t1lib --disable-gd-jis-conv --without-jpeg-dir --without-png-dir --without-xpm-dir --without-vpx-dir --without-gd --without-interbase --with-mysql=mysqlnd --with-mysqli=mysqlnd --with-mysql-sock=/var/run/mysqld/mysqld.sock --without-unixODBC --without-iodbc --without-oci8 --without-pdo-dblib --with-pdo-mysql=mysqlnd --without-pdo-pgsql --without-pdo-sqlite --without-pdo-firebird --without-pdo-odbc --without-pdo-oci --with-readline=/usr --without-libedit --without-mm --with-pic --with-pcre-regex=/usr --with-pcre-dir=/usr --cache-file=/var/tmp/portage/dev-lang/php-5.6.31/temp/config.cache --with-config-file-path=/etc/php/cli-php5.6 --with-config-file-scan-dir=/etc/php/cli-php5.6/ext-active --disable-embed --enable-cli --disable-cgi --disable-fpm --without-apxs2

Seems that problem is related to child spawn, the lower the *pm.max_requests* value, the more likely the problem to appear.

Currently working on getting a good backtrace

The stack trace looks similar to other bugs from "use after free" family:
Bug #77114 	php-fpm master segfaults in fpm_event_epoll_wait/fpm_event_fire
(see comments for a patch),
Bug #77185 	Use-after-free in FPM master event handling
and several others

Well, the other bugs should have been fixed in the meantime, so I
wonder if anybody can still reproduce this with any of the
actively supported PHP versions[1]?

[1] <https://www.php.net/supported-versions.php>

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