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

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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