linux-kernel - Re: [PATCH v4 2/2] livepatch: force transition to finish

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Message-ID: <nycvar.YFH.7.76.1711210004460.32604@cbobk.fhfr.pm>
Date:   Tue, 21 Nov 2017 00:09:23 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     Miroslav Benes <mbenes@...e.cz>,
        Josh Poimboeuf <jpoimboe@...hat.com>
cc:     jeyu@...nel.org, pmladek@...e.com, lpechacek@...e.cz, pavel@....cz,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] livepatch: force transition to finish

On Mon, 20 Nov 2017, Miroslav Benes wrote:

> While working on "immediate" removal, I realized we had the similar 
> problem here with modules removal. There is no way out of the rabbit hole.
> 
> If a patch is forced, we obviously cannot say there is no task sleeping in 
> the old code. This could be disastrous if such old module is then removed 
> (either we disabled it and we want to rmmod it, or there is a new "atomic 
> replace" patch and we want to remove the old one).
> 
> We need something like the following (at least as a starting point)

I agree; the only thing I think really has to be done is putting a comment 
there, explaining why forcing implies infinite module reference (and also 
perhaps making it therefore even more obvious from documentation, that 
this really is a last-resort-"you-know-what-you-are-doing" kind of knob).

On Mon, 20 Nov 2017, Josh Poimboeuf wrote:

> > We can also try to improve later. We could remember all forced tasks
> > and reenable rmmod once those tasks are really migrated ("shadow
> > migration"). 
> 
> NACK :-)  Forcing should hopefully be a rare event, not worth the
> trouble to try to keep track of that IMO.

Well, that was my rather random idea when we were discussing this over 
lunch today. But I agree, it definitely is a total overkill, and I don't 
want it to be atributed to me any more :p

Thanks,

-- 
Jiri Kosina
SUSE Labs