• Lennart Poettering's avatar
    prctl: add PR_{SET,GET}_CHILD_SUBREAPER to allow simple process supervision · ebec18a6
    Lennart Poettering authored
    Userspace service managers/supervisors need to track their started
    services.  Many services daemonize by double-forking and get implicitly
    re-parented to PID 1.  The service manager will no longer be able to
    receive the SIGCHLD signals for them, and is no longer in charge of
    reaping the children with wait().  All information about the children is
    lost at the moment PID 1 cleans up the re-parented processes.
    
    With this prctl, a service manager process can mark itself as a sort of
    'sub-init', able to stay as the parent for all orphaned processes
    created by the started services.  All SIGCHLD signals will be delivered
    to the service manager.
    
    Receiving SIGCHLD and doing wait() is in cases of a service-manager much
    preferred over any possible asynchronous notification about specific
    PIDs, because the service manager has full access to the child process
    data in /proc and the PID can not be re-used until the wait(), the
    service-manager itself is in charge of, has happened.
    
    As a side effect, the relevant parent PID information does not get lost
    by a double-fork, which results in a more elaborate process tree and
    'ps' output:
    
    before:
      # ps afx
      253 ?        Ss     0:00 /bin/dbus-daemon --system --nofork
      294 ?        Sl     0:00 /usr/libexec/polkit-1/polkitd
      328 ?        S      0:00 /usr/sbin/modem-manager
      608 ?        Sl     0:00 /usr/libexec/colord
      658 ?        Sl     0:00 /usr/libexec/upowerd
      819 ?        Sl     0:00 /usr/libexec/imsettings-daemon
      916 ?        Sl     0:00 /usr/libexec/udisks-daemon
      917 ?        S      0:00  \_ udisks-daemon: not polling any devices
    
    after:
      # ps afx
      294 ?        Ss     0:00 /bin/dbus-daemon --system --nofork
      426 ?        Sl     0:00  \_ /usr/libexec/polkit-1/polkitd
      449 ?        S      0:00  \_ /usr/sbin/modem-manager
      635 ?        Sl     0:00  \_ /usr/libexec/colord
      705 ?        Sl     0:00  \_ /usr/libexec/upowerd
      959 ?        Sl     0:00  \_ /usr/libexec/udisks-daemon
      960 ?        S      0:00  |   \_ udisks-daemon: not polling any devices
      977 ?        Sl     0:00  \_ /usr/libexec/packagekitd
    
    This prctl is orthogonal to PID namespaces.  PID namespaces are isolated
    from each other, while a service management process usually requires the
    services to live in the same namespace, to be able to talk to each
    other.
    
    Users of this will be the systemd per-user instance, which provides
    init-like functionality for the user's login session and D-Bus, which
    activates bus services on-demand.  Both need init-like capabilities to
    be able to properly keep track of the services they start.
    
    Many thanks to Oleg for several rounds of review and insights.
    
    [akpm@linux-foundation.org: fix comment layout and spelling]
    [akpm@linux-foundation.org: add lengthy code comment from Oleg]
    Reviewed-by: default avatarOleg Nesterov <oleg@redhat.com>
    Signed-off-by: default avatarLennart Poettering <lennart@poettering.net>
    Signed-off-by: default avatarKay Sievers <kay.sievers@vrfy.org>
    Acked-by: default avatarValdis Kletnieks <Valdis.Kletnieks@vt.edu>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    ebec18a6
fork.c 43.4 KB