[PATCH] sysrq: Use SEND_SIG_FORCED instead of force_sig()

[Anton Vorontsov <anton.vorontsov@linaro.org>]

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);
 }
-- 
1.7.9.2