Hey, I’ve hit this exact frustrating issue before when working with Linux kernel static keys! Let’s break down why this happens and how to fix it, especially since the problem only shows up with the jump label implementation:

Key Background: Jump Label vs. unlikely() Fallback #

First, let’s clarify the two implementations of static_key_false:

• Jump Label (default when CONFIG_JUMP_LABEL is enabled): Uses kernel dynamic patching to rewrite branch instructions at runtime. It’s fast, but the patch application is asynchronous—meaning the change doesn’t take effect immediately.
• unlikely() Fallback: When jump labels are disabled, it falls back to a direct runtime check of the key’s counter. This is slightly less performant but reflects changes instantly.

Your issue is rooted in the asynchronous nature of the jump label path—your code is checking the key before the kernel finishes applying the patch triggered by static_key_slow_inc.

Common Causes & Fixes #

1. Incorrect Static Key Initialization #

This is the easiest mistake to make: never manually declare the static key struct without using the official initialization macros. If you skip this, the jump label infrastructure won’t recognize the key, so static_key_slow_inc won’t trigger any patching at all.

Correct initialization examples:

// Global scope
DEFINE_STATIC_KEY_FALSE(my_static_key);

// Static/local scope
static struct static_key my_static_key = STATIC_KEY_INIT_FALSE;

2. Asynchronous Patching Delay #

static_key_slow_inc() doesn’t apply the patch right away—it only marks the key for update. The actual code patching happens later via RCU (Read-Copy Update) synchronization. If you check the key immediately after calling static_key_slow_inc, the patch might not have run yet.

If your code runs in process context (not interrupt/atomic context), wait for RCU synchronization to ensure the patch is active:

static_key_slow_inc(&my_static_key);

// Wait for RCU to finish applying the patch (blocks, so no interrupts!)
if (!in_interrupt()) {
    synchronize_rcu();
}

// Now static_key_false should recognize the updated state
if (static_key_false(&my_static_key)) {
    // Your code here will trigger as expected
}

3. Early Boot Context Limitations #

If you’re modifying the key during early kernel boot (before the RCU or jump label infrastructure is fully initialized), the patching mechanism won’t work. In this case:

• Delay modifying the key until after the system is fully booted, or
• Use static_key_enabled(&my_static_key) instead of static_key_false—this bypasses jump labels and checks the key’s counter directly, just like the unlikely() fallback.

4. Compiler Optimization Edge Case #

Rarely, aggressive compiler optimizations might inline the static_key_false check in a way that bypasses the jump label hook. To rule this out:

• Add __attribute__((noinline)) to the function containing the check, or
• Upgrade to a more recent kernel version (newer releases have better jump label compiler support).

Alternative: Immediate Runtime Check #

If you can’t wait for RCU synchronization (e.g., in interrupt context), use static_key_enabled() for your check. It’s slightly slower, but it reflects the key’s state instantly:

static_key_slow_inc(&my_static_key);
if (static_key_enabled(&my_static_key)) {
    // This will work immediately
}

内容的提问来源于stack exchange，提问作者tomer.z