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    Subject[PATCH 2/4] pids: init_struct_pid.tasks should never see the swapper process
    "statically initialize struct pid for swapper" commit 820e45db says:

    Statically initialize a struct pid for the swapper process (pid_t == 0)
    and attach it to init_task. This is needed so task_pid(), task_pgrp()
    and task_session() interfaces work on the swapper process also.

    OK, but:

    - it doesn't make sense to add init_task.pids[].node into
    init_struct_pid.tasks[], and in fact this just wrong.

    idle threads are special, they shouldn't be visible on any
    global list. In particular do_each_pid_task(init_struct_pid)
    shouldn't see swapper.

    This is the actual reason why kill(0, SIGKILL) from /sbin/init
    (which starts with 0,0 special pids) crashes the kernel. The
    signal sent to pgid/sid == 0 must never see idle threads, even
    if the previous patch fixed the crash itself.

    - we have other idle threads running on the non-boot CPUs, see
    the next patch.

    Change INIT_STRUCT_PID/INIT_PID_LINK to create the empty/unhashed
    hlist_head/hlist_node. Like any other idle thread swapper can never exit,
    so detach_pid()->__hlist_del() is not possible, but we could change
    INIT_PID_LINK() to set pprev = &next if needed.

    All we need is the valid swapper->pids[].pid == &init_struct_pid.

    Reported-by: Mathias Krause <mathias.krause@secunet.com>
    Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    ---

    include/linux/init_task.h | 8 ++++----
    1 file changed, 4 insertions(+), 4 deletions(-)

    --- 34-rc1/include/linux/init_task.h~2_INIT_PID_DONT_LINK_SWAPPER 2010-05-10 19:45:27.000000000 +0200
    +++ 34-rc1/include/linux/init_task.h 2010-05-10 20:26:08.000000000 +0200
    @@ -53,9 +53,9 @@ extern struct group_info init_groups;
    #define INIT_STRUCT_PID { \
    .count = ATOMIC_INIT(1), \
    .tasks = { \
    - { .first = &init_task.pids[PIDTYPE_PID].node }, \
    - { .first = &init_task.pids[PIDTYPE_PGID].node }, \
    - { .first = &init_task.pids[PIDTYPE_SID].node }, \
    + { .first = NULL }, \
    + { .first = NULL }, \
    + { .first = NULL }, \
    }, \
    .rcu = RCU_HEAD_INIT, \
    .level = 0, \
    @@ -70,7 +70,7 @@ extern struct group_info init_groups;
    { \
    .node = { \
    .next = NULL, \
    - .pprev = &init_struct_pid.tasks[type].first, \
    + .pprev = NULL, \
    }, \
    .pid = &init_struct_pid, \
    }


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