• Pavel Tikhomirov's avatar
    prctl: propagate has_child_subreaper flag to every descendant · 749860ce
    Pavel Tikhomirov authored
    If process forks some children when it has is_child_subreaper
    flag enabled they will inherit has_child_subreaper flag - first
    group, when is_child_subreaper is disabled forked children will
    not inherit it - second group. So child-subreaper does not reparent
    all his descendants when their parents die. Having these two
    differently behaving groups can lead to confusion. Also it is
    a problem for CRIU, as when we restore process tree we need to
    somehow determine which descendants belong to which group and
    much harder - to put them exactly to these group.
    
    To simplify these we can add a propagation of has_child_subreaper
    flag on PR_SET_CHILD_SUBREAPER, walking all descendants of child-
    subreaper to setup has_child_subreaper flag.
    
    In common cases when process like systemd first sets itself to
    be a child-subreaper and only after that forks its services, we will
    have zero-length list of descendants to walk. Testing with binary
    subtree of 2^15 processes prctl took < 0.007 sec and has shown close
    to linear dependency(~0.2 * n * usec) on lower numbers of processes.
    
    Moreover, I doubt someone intentionaly pre-forks the children whitch
    should reparent to init before becoming subreaper, because some our
    ancestor migh have had is_child_subreaper flag while forking our
    sub-tree and our childs will all inherit has_child_subreaper flag,
    and we have no way to influence it. And only way to check if we have
    no has_child_subreaper flag is to create some childs, kill them and
    see where they will reparent to.
    
    Using walk_process_tree helper to walk subtree, thanks to Oleg! Timing
    seems to be the same.
    
    Optimize:
    
    a) When descendant already has has_child_subreaper flag all his subtree
    has it too already.
    
    * for a) to be true need to move has_child_subreaper inheritance under
    the same tasklist_lock with adding task to its ->real_parent->children
    as without it process can inherit zero has_child_subreaper, then we
    set 1 to it's parent flag, check that parent has no more children, and
    only after child with wrong flag is added to the tree.
    
    * Also make these inheritance more clear by using real_parent instead of
    current, as on clone(CLONE_PARENT) if current has is_child_subreaper
    and real_parent has no is_child_subreaper or has_child_subreaper, child
    will have has_child_subreaper flag set without actually having a
    subreaper in it's ancestors.
    
    b) When some descendant is child_reaper, it's subtree is in different
    pidns from us(original child-subreaper) and processes from other pidns
    will never reparent to us.
    
    So we can skip their(a,b) subtree from walk.
    
    v2: switch to walk_process_tree() general helper, move
    has_child_subreaper inheritance
    v3: remove csr_descendant leftover, change current to real_parent
    in has_child_subreaper inheritance
    v4: small commit message fix
    
    Fixes: ebec18a6 ("prctl: add PR_{SET,GET}_CHILD_SUBREAPER to allow simple process supervision")
    Signed-off-by: default avatarPavel Tikhomirov <ptikhomirov@virtuozzo.com>
    Reviewed-by: default avatarOleg Nesterov <oleg@redhat.com>
    Signed-off-by: default avatarEric W. Biederman <ebiederm@xmission.com>
    749860ce
fork.c 56.7 KB