Re: static_branch_enable() does not work from a __init function?

[Jessica Yu]

+++ Peter Zijlstra [16/12/20 13:47 +0100]:
> On Wed, Dec 16, 2020 at 12:55:25PM +0100, Jessica Yu wrote:
> > +++ Peter Zijlstra [16/12/20 10:26 +0100]:
> > [snip]
> > > > PS, I originally found: in arch/x86/kvm/vmx/vmx.c: vmx_init(), it looks
> > > > like the line "static_branch_enable(&enable_evmcs);" does not take effect
> > > > in a v5.4-based kernel, but does take effect in the v5.10 kernel in the
> > > > same x86-64 virtual machine on Hyper-V, so I made the above test module
> > > > to test static_branch_enable(), and found that static_branch_enable() in
> > > > the test module does not work with both v5.10 and my v5.4 kernel, if the
> > > > __init marker is used.
> >
> > Because the jump label code currently does not allow you to update if
> > the entry resides in an init section. By marking the module init
> > section __init you place it in the .init.text section.
> > jump_label_add_module() detects this (by calling within_module_init())
> > and marks the entry by calling jump_entry_set_init(). Then you have
> > the following sequence of calls (roughly):
> >
> > static_branch_enable
> >  static_key_enable
> >    static_key_enable_cpuslocked
> >      jump_label_update
> >        jump_label_can_update
> >          jump_entry_is_init returns true, so bail out
> >
> > Judging from the comment in jump_label_can_update(), this seems to be
> > intentional behavior:
> >
> > static bool jump_label_can_update(struct jump_entry *entry, bool init)
> > {
> >        /*
> >         * Cannot update code that was in an init text area.
> >         */
> >        if (!init && jump_entry_is_init(entry))
> >                return false;
>
> Only because we're having .init=false, incorrectly. See the other email.

Ah yeah, you're right. I also misread the intention of the if
conditional :/ If we're currently running an init function it's fine,
but after that it will be unsafe.

Btw, your patch seems to work for me, using the test module provided
by Dexuan.