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

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Date:   Tue, 21 Nov 2017 11:04:24 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     jpoimboe@...hat.com, jeyu@...nel.org, jikos@...nel.org,
        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 2017-11-20 16:57:19, Miroslav Benes wrote:
> On Wed, 15 Nov 2017, Miroslav Benes wrote:
> 
> > If a task sleeps in a set of patched functions uninterruptedly, it could
> > block the whole transition indefinitely.  Thus it may be useful to clear
> > its TIF_PATCH_PENDING to allow the process to finish.
> > 
> > Admin can do that now by writing to force sysfs attribute in livepatch
> > sysfs directory. TIF_PATCH_PENDING is then cleared for all tasks and the
> > transition can finish successfully.
> > 
> > Important note! Administrator should not use this feature without a
> > clearance from a patch distributor. It must be checked that by doing so
> > the consistency model guarantees are not violated.
> > 
> > Signed-off-by: Miroslav Benes <mbenes@...e.cz>
> 
> 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)

Great catch! The proposed solution looks good to me. And as already
mentioned, it might be worth updating the documentation.

For the whole patchset, including this fixup:

Reviewed-by: Petr Mladek <pmladek@...e.com>

Best Regards,
Petr