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Date: Mon, 26 Mar 2012 15:43:03 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Anton Vorontsov <anton.vorontsov@linaro.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Oleg Nesterov <oleg@redhat.com>, Alan Cox <alan@linux.intel.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sysrq: Use SEND_SIG_FORCED instead of force_sig()
Message-Id: <20120326154303.86126785.akpm@linux-foundation.org>
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On Sat, 24 Mar 2012 15:00:24 +0400
Anton Vorontsov <anton.vorontsov@linaro.org> 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.
> 
> Some more notes from Oleg Nesterov:
> 
> > Just one note. This change makes no difference for sysrq_handle_kill().
> > But it obviously changes the behaviour sysrq_handle_term(). I think
> > this is fine, if you want to really kill the task which blocks/ignores
> > SIGTERM you can use sysrq_handle_kill().
> >
> > Even ignoring the reasons why force_sig() is simply wrong here,
> > force_sig(SIGTERM) looks strange. The task won't be killed if it has
> > a handler, but SIG_IGN can't help. However if it has the handler
> > but blocks SIGTERM temporary (this is very common) it will be killed.
> 
> Also,
> 
> > force_sig() can't kill the process if the main thread has already
> > exited. IOW, it is trivial to create the process which can't be
> > killed by sysrq.
> 
> So, this patch fixes the issue.
> 
> Suggested-by: Oleg Nesterov <oleg@redhat.com>
> Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Acked-by: Oleg Nesterov <oleg@redhat.com>
> Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
> ---
> 
> The patch depends on a few Oleg's patches in -mm, so I believe
> this should be -mm material as well.
> 
>  drivers/tty/sysrq.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
> index 8db9125..5ab8039 100644
> --- a/drivers/tty/sysrq.c
> +++ b/drivers/tty/sysrq.c
> @@ -329,7 +329,7 @@ static void send_sig_all(int sig)
>  		if (is_global_init(p))
>  			continue;
>  
> -		force_sig(sig, p);
> +		do_send_sig_info(sig, SEND_SIG_FORCED, p, true);
>  	}
>  	read_unlock(&tasklist_lock);
>  }

It's unclear how serious this race is (I'm guessing "not very"), but
this patch looks like 3.3 material anyway, yes?