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

[Anton Vorontsov]

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@xxxxxxxxxx>
Signed-off-by: Anton Vorontsov <anton.vorontsov@xxxxxxxxxx>
---

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?

 drivers/tty/sysrq.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

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);
 }
-- 
1.7.7.6

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