sometimes zabbix agent will leave one process hanging upon exit. this has so far always happened after sending "killall -15 zabbix_agentd".
strace shows :
Process 10468 attached - interrupt to quit
futex(0xb783ca50, FUTEX_WAIT_PRIVATE, 2, NULL
what's confusing, this pid does not appear in the agentd log at all.
no idea whether this is in any way related to shared memory/semaphores, but the only thing owned by zabbix at that time is :
------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x00000000 3801088 zabbix 600 491612 1 dest