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

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Date:   Wed, 16 Dec 2020 10:26:49 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dexuan Cui <decui@...rosoft.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        jeyu@...nel.org, Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: static_branch_enable() does not work from a __init function?

On Wed, Dec 16, 2020 at 03:54:29AM +0000, Dexuan Cui wrote:
> 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?

*groan*... I think this is because __init is ran with
MODULE_STATE_COMING, we only switch to MODULE_STATE_LIVE later.

Let me see if there's a sane way to untangle that.

> #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
> 
>