Re: [PATCH 6/9] livepatch/ftrace: Add recursion protection to the ftrace callback

Miroslav Benes
2020-10-30T09:49:02Z

> > > > + bit = ftrace_test_recursion_trylock();
> > > > + if (bit < 0)
> > > > + return;
> > >
> > > This means that the original function will be called in case of recursion.
> > > That's probably fair, but I'm wondering if we should at least WARN about
> > > it.
> >
> > Yeah, the early return might break the consistency model and
> > unexpected things might happen. We should be aware of it.
> > Please use:
> >
> > if (WARN_ON_ONCE(bit < 0))
> > return;
> >
> > WARN_ON_ONCE() might be part of the recursion. But it should happen
> > only once. IMHO, it is worth the risk.
> >
> > Otherwise it looks good.
>
> Perhaps we can add that as a separate patch, because this patch doesn't add
> any real functionality change. It only moves the recursion testing from the
> helper function (which ftrace wraps all callbacks that do not have the
> RECURSION flags set, including this one) down to your callback.
>
> In keeping with one patch to do one thing principle, the added of
> WARN_ON_ONCE() should be a separate patch, as that will change the
> functionality.
>
> If that WARN_ON_ONCE() breaks things, I'd like it to be bisected to another
> patch other than this one.

Works for me.

Miroslav