Linux Permissions – Why Can’t Other Users Remove /dev/shm/xxx?

I have python scripts that creates shared semaphore /dev/shm/sem.serialize_ld with permissions

$ ll /dev/shm/sem.serialize_ld                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
-rw-rw-rw- 1 buildbot buildbot 32 mar 27 08:36 /dev/shm/sem.serialize_ld

In practice it is always created by user buildbot, but it is not design assumption.
Sometimes problem happens – script is killed by SIGKILL and doesn't release semaphore. New invocations deadlock on start. To manually fix this situation I want to remove the file and start over. However only buildbot and root users can do this.

Why can't different user remove this file?

$ rm /dev/shm/sem.serialize_ld                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
rm: cannot remove '/dev/shm/sem.serialize_ld': Operation not permitted

System is Ubuntu 18.04.2 LTS

Script code is:

#!/usr/bin/env python3                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
# -*- coding: utf-8 -*-                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            

import posix_ipc                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
import subprocess                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
import sys                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
import signal                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      

SEM_NAME = '/serialize_ld'                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
MAX_CONCURRENT = 2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
PROGRAM = '/usr/bin/ld'                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            


def main():                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
    import os                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
    os.umask(0)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
    sem = posix_ipc.Semaphore(SEM_NAME, posix_ipc.O_CREAT, mode=0o666, initial_value=MAX_CONCURRENT)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
    sem.acquire()                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  

    def release_semaphore(signum, frame):                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          
        print("exiting due to signal " + str(signum))                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
        sem.release()                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
        sem.close()                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
        sys.exit(1)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
    signal.signal(signal.SIGTERM | signal.SIGINT, release_semaphore)                                                                                                                                                                                                                                                                                                                                                                                                                                                              

    try:                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
        subprocess.call([PROGRAM, *sys.argv[1:]])                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
    finally:
        sem.release()
        sem.close()


if __name__ == "__main__":
    main()