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    SubjectRe: [PATCH] sysrq: Use SEND_SIG_FORCED instead of force_sig()
    On Wed, Feb 15, 2012 at 02:50:17AM +0400, Anton Vorontsov wrote:
    > Change send_sig_all() to use do_send_sig_info(SEND_SIG_FORCED)
    > instead of force_sig(SIGKILL). With the recent changes we do not
    > need force_ to kill the CLONE_NEWPID tasks.
    >
    > And this is more correct. force_sig() can race with the exiting
    > thread, while do_send_sig_info(group => true) kill the whole
    > process.
    >
    > Suggested-by: Oleg Nesterov <oleg@redhat.com>
    > Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
    > ---
    >
    > On Fri, Feb 10, 2012 at 09:10:08PM +0100, Oleg Nesterov wrote:
    > > > --- a/drivers/tty/sysrq.c
    > > > +++ b/drivers/tty/sysrq.c
    > > > @@ -324,9 +324,12 @@ static void send_sig_all(int sig)
    > > >
    > > > read_lock(&tasklist_lock);
    > > > for_each_process(p) {
    > > > - if (p->mm && !is_global_init(p))
    > > > - /* Not swapper, init nor kernel thread */
    > > > - force_sig(sig, p);
    > > > + if (p->flags & PF_KTHREAD)
    > > > + continue;
    > > > + if (is_global_init(p))
    > > > + continue;
    > > > +
    > > > + force_sig(sig, p);
    > > > }
    > > > read_unlock(&tasklist_lock);
    > >
    > > Obviously I agree with this change.
    > >
    > > But where does this read_lock(tasklist) come from?
    >
    > It came from this patch: http://lkml.org/lkml/2012/2/7/24
    >
    > > We discussed this with Anton. Yes, tasklist ensures that
    > > force_sig() can't crash the kernel. But it is still wrong
    > > and should not be used.
    > >
    > > I think send_sig_all() should use SEND_SIG_FORCED (this
    > > depends on the patches I sent to Andrew), in this case
    > > tasklist is not needed.
    >
    > Well, I think the lock is still a good thing: we don't want
    > any new processes to be created while we kill others.
    >
    > I might be wrong, but copy_process() issues recalc_sigpending()
    > under tasklist lock especially the for this scenario.
    >
    > So, in this and in OOM cases we have to be precise (unlike LMK).
    > Sysrq is a rare thing, so there is actually should be no problem
    > with holding the lock.
    >
    > So, how about this patch?
    >
    > Greg, can we take it via -mm tree, as it depends on a few
    > sched patches?

    That's fine with me:
    Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>



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