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    Subject[PATCH] change kernel threads to ignore signals instead of blocking them
    On top of Eric's

    kthread-dont-depend-on-work-queues-take-2.patch

    Currently kernel threads use sigprocmask(SIG_BLOCK) to protect against signals.
    This doesn't prevent the signal delivery, this only blocks signal_wake_up().
    Every "killall -33 kthreadd" means a "struct siginfo" leak.

    Change kthreadd_setup() to set all handlers to SIG_IGN instead of blocking them
    (make a new helper ignore_signals() for that). If the kernel thread needs some
    signal, it should use allow_signal() anyway, and in that case it should not use
    CLONE_SIGHAND.

    Note that we can't change daemonize() (should die!) in the same way, because
    it can be used along with CLONE_SIGHAND. This means that allow_signal() still
    should unblock the signal to work correctly with daemonize()ed threads.

    However, disallow_signal() doesn't block the signal any longer but ignores it.

    NOTE: with or without this patch the kernel threads are not protected from
    handle_stop_signal(), this seems harmless, but not good.

    Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

    include/linux/sched.h | 1 +
    kernel/exit.c | 2 +-
    kernel/kthread.c | 17 +++--------------
    kernel/signal.c | 10 ++++++++++
    4 files changed, 15 insertions(+), 15 deletions(-)

    --- 2.6.21-rc5/include/linux/sched.h~1_SIGIGN 2007-04-05 12:18:28.000000000 +0400
    +++ 2.6.21-rc5/include/linux/sched.h 2007-04-13 00:09:56.000000000 +0400
    @@ -1299,6 +1299,7 @@ extern int in_egroup_p(gid_t);

    extern void proc_caches_init(void);
    extern void flush_signals(struct task_struct *);
    +extern void ignore_signals(struct task_struct *);
    extern void flush_signal_handlers(struct task_struct *, int force_default);
    extern int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info);

    --- 2.6.21-rc5/kernel/signal.c~1_SIGIGN 2007-04-05 12:18:28.000000000 +0400
    +++ 2.6.21-rc5/kernel/signal.c 2007-04-13 02:14:06.000000000 +0400
    @@ -329,6 +329,16 @@ void flush_signals(struct task_struct *t
    spin_unlock_irqrestore(&t->sighand->siglock, flags);
    }

    +void ignore_signals(struct task_struct *t)
    +{
    + int i;
    +
    + for (i = 0; i < _NSIG; ++i)
    + t->sighand->action[i].sa.sa_handler = SIG_IGN;
    +
    + flush_signals(t);
    +}
    +
    /*
    * Flush all handlers for a task.
    */
    --- 2.6.21-rc5/kernel/kthread.c~1_SIGIGN 2007-04-12 23:18:09.000000000 +0400
    +++ 2.6.21-rc5/kernel/kthread.c 2007-04-13 02:27:39.000000000 +0400
    @@ -215,24 +215,13 @@ EXPORT_SYMBOL(kthread_stop);
    static __init void kthreadd_setup(void)
    {
    struct task_struct *tsk = current;
    - struct k_sigaction sa;
    - sigset_t blocked;

    set_task_comm(tsk, "kthreadd");

    - /* Block and flush all signals */
    - sigfillset(&blocked);
    - sigprocmask(SIG_BLOCK, &blocked, NULL);
    - flush_signals(tsk);
    -
    - /* SIG_IGN makes children autoreap: see do_notify_parent(). */
    - sa.sa.sa_handler = SIG_IGN;
    - sa.sa.sa_flags = 0;
    - siginitset(&sa.sa.sa_mask, sigmask(SIGCHLD));
    - do_sigaction(SIGCHLD, &sa, (struct k_sigaction *)0);
    + ignore_signals(tsk);

    - set_user_nice(current, -5);
    - set_cpus_allowed(current, CPU_MASK_ALL);
    + set_user_nice(tsk, -5);
    + set_cpus_allowed(tsk, CPU_MASK_ALL);
    }

    int kthreadd(void *unused)
    --- 2.6.21-rc5/kernel/exit.c~1_SIGIGN 2007-04-12 23:23:50.000000000 +0400
    +++ 2.6.21-rc5/kernel/exit.c 2007-04-13 10:17:06.000000000 +0400
    @@ -348,7 +348,7 @@ int disallow_signal(int sig)
    return -EINVAL;

    spin_lock_irq(&current->sighand->siglock);
    - sigaddset(&current->blocked, sig);
    + current->sighand->action[(sig)-1].sa.sa_handler = SIG_IGN;
    recalc_sigpending();
    spin_unlock_irq(&current->sighand->siglock);
    return 0;
    -
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