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

[Jessica Yu <jeyu@kernel.org>]

+++ 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.