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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
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