linux-kernel - Re: [RFC PATCH 06/18] signal/kthread: Initial implementation of kthread signal handling

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Date:	Mon, 15 Jun 2015 21:14:29 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Petr Mladek <pmladek@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Richard Weinberger <richard@....at>,
	Steven Rostedt <rostedt@...dmis.org>,
	David Woodhouse <dwmw2@...radead.org>,
	linux-mtd@...ts.infradead.org,
	Trond Myklebust <trond.myklebust@...marydata.com>,
	Anna Schumaker <anna.schumaker@...app.com>,
	linux-nfs@...r.kernel.org, Chris Mason <clm@...com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>, Borislav Petkov <bp@...e.de>,
	Michal Hocko <mhocko@...e.cz>, live-patching@...r.kernel.org,
	linux-api@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 06/18] signal/kthread: Initial implementation of
	kthread signal handling

Hi Petr,

On 06/15, Petr Mladek wrote:
>
> I am sorry for the late reply. I wanted to think more before answering
> all the mails.

Don't worry I am always late ;)

> On Mon 2015-06-08 23:13:36, Oleg Nesterov wrote:
> > >
> > > Hmm, the helper would have a strange semantic. You need to take
> > > sighand->siglock, dequeue the signal (SIGSTOP), and call
> > > __set_current_state(TASK_STOPPED) before you release the lock.
> > > But what would happen if the dequeued signal is _not_ SIGSTOP?
> >
> > Perhaps I missed your point, but no. If you want to handle SIGSTOP
> > you can do
> >
>
> I think that we need to add:
>
> 	spin_lock_irq(&sighand->siglock);
>
> > 	signr = kthread_signal_dequeue();
> > 	switch (signr) {
> > 	case SIGSTOP:
> > 		something_else();
> > 		kthread_do_signal_stop();
> > 	...
> > 	}
>
> And if we want to avoid any race, kthread_do_signal_stop() should look like:
>
> void kthread_do_signal_stop(unsigned long flags)
> {
> 	struct sighand_struct *sighand = current->sighand;
>
> 	__set_current_state(TASK_STOPPED);
> 	spin_unlock_irqrestore(&sighand->siglock, flags);
> 	/* Don't run again until woken by SIGCONT or SIGKILL */
> 	freezable_schedule();
> }

Ah, understand. You think that we need to take ->siglock in advance
to avoid the race with SIGCONT?

No, we don't. Let me show you the code I suggested again:

	void kthread_do_signal_stop(void)
	{
		spin_lock_irq(&curtent->sighand->siglock);
		if (current->jobctl & JOBCTL_STOP_DEQUEUED)
			__set_current_state(TASK_STOPPED);
		spin_unlock_irq(&current->sighand->siglock);

		schedule();
	}
	
so you can dequeue_signal() and call kthread_do_signal_stop() without
holding ->siglock. We can rely on JOBCTL_STOP_DEQUEUED bit. SIGCONT
clears it, so kthread_do_signal_stop() can't race.

Oleg.

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