linux-kernel - Re: [PATCH 1/2] livepatch: Send a fake signal periodically

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Date:   Tue, 5 Jun 2018 09:01:41 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     jikos@...nel.org, jeyu@...nel.org, pmladek@...e.com,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] livepatch: Send a fake signal periodically

On Tue, Jun 05, 2018 at 09:17:52AM +0200, Miroslav Benes wrote:
> On Mon, 4 Jun 2018, Josh Poimboeuf wrote:
> 
> > On Mon, Jun 04, 2018 at 04:16:35PM +0200, Miroslav Benes wrote:
> > > An administrator may send a fake signal to all remaining blocking tasks
> > > of a running transition by writing to
> > > /sys/kernel/livepatch/<patch>/signal attribute. Let's do it
> > > automatically after 10 seconds. The timeout is chosen deliberately. It
> > > gives the tasks enough time to transition themselves.
> > > 
> > > Theoretically, sending it once should be more than enough. Better be safe
> > > than sorry, so send it periodically.
> > 
> > This is the part I don't understand.  Why do it periodically?
> 
> I met (rare!) cases when doing it once was not enough due to a race and 
> the signal was missed. However involved testcases were really artificial.
>  
> > Instead, might it make sense to just send the signals once, and if that
> > doesn't work, reverse the transition?  Then we could make patching a
> > synchronous operation.  But then, it might be remotely possible that the
> > reverse operation also stalls (e.g., on a kthread).  So, maybe it's best
> > to just leave all these controls in the hands of the user.
> 
> And there is 'force' option...
> 
> So given all this, I'd call klp_send_signals() once and then leave it up 
> to the user. Would that work for you?

Well, I don't know.  Since the patching process will already need to be
managed by user space, what's the benefit of having the kernel doing
only this part of it?

-- 
Josh