linux-kernel - static_branch_enable() does not work from a _

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Date:   Wed, 16 Dec 2020 03:54:29 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: static_branch_enable() does not work from a __init function?

Hi,
The below init_module() prints "foo: false". This is strange since
static_branch_enable() is called before the static_branch_unlikely().
This strange behavior happens to v5.10 and an old v5.4 kernel.

If I remove the "__init" marker from the init_module() function, then
I get the expected output of "foo: true"! I guess here I'm missing
something with Static Keys?

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/jump_label.h>

static DEFINE_STATIC_KEY_FALSE(enable_foo);

int __init init_module(void)
{
        static_branch_enable(&enable_foo);

        if (static_branch_unlikely(&enable_foo))
                printk("foo: true\n");
        else
                printk("foo: false\n");

        return 0;
}

void cleanup_module(void)
{
        static_branch_disable(&enable_foo);
}

MODULE_LICENSE("GPL");

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.

Thanks,
-- Dexuan