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

[Greg Kroah-Hartman]

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@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?

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

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