[PATCH v2 20/20] jump_label: RFC - tolerate toggled state

[Peter Zijlstra]

On Fri, Jan 13, 2023 at 12:30:16PM -0700, Jim Cromie wrote:
> __jump_label_patch currently will "crash the box" if it finds a
> jump_entry not as expected.  ISTM this overly harsh; it doesn't
> distinguish between "alternate/opposite" state, and truly
> "insane/corrupted".
> 
> The "opposite" (but well-formed) state is a milder mis-initialization
> problem, and some less severe mitigation seems practical.  ATM this
> just warns about it; a range/enum of outcomes: warn, crash, silence,
> ok, fixup-continue, etc, are possible on a case-by-case basis.
> 
> Ive managed to create this mis-initialization condition with
> test_dynamic_debug.ko & _submod.ko.  These replicate DRM's regression
> on DRM_USE_DYNAMIC_DEBUG=y; drm.debug callsites in drivers/helpers
> (dependent modules) are not enabled along with those in drm.ko itself.
> 

> Ive hit this case a few times, but havent been able to isolate the
> when and why.
> 
> warn-only is something of a punt, and I'm still left with remaining
> bugs which are likely related; I'm able to toggle the p-flag on
> callsites in the submod, but their enablement still doesn't yield
> logging activity.

Right; having been in this is state is bad since it will generate
inconsistent code-flow. Full on panic *might* not be warranted (as it
does for corrupted text) but it is still a fairly bad situation -- so
I'm not convinced we want to warn and carry on.

It would be really good to figure out why the site was skipped over and
got out of skew.

Given it's all module stuff, the 'obvious' case would be something like
a race between adding the new sites and flipping it, but I'm not seeing
how -- things are rather crudely serialized by jump_label_mutex.

The only other option I can come up with is that somehow the update
condition in jump_label_add_module() is somehow wrong.