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

[Jessica Yu]

+++ 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;

By removing the __init marker you're bypassing the
within_module_init() check and that's why it works.