ASSERT_EXCLUSIVE_WRITER(9)

📦 version 7.3.1, in nixpkgs unstable

💅 ASSERT_EXCLUSIVE_WRITER(9) — your fave kernel API manual bestie ✨

okay so before we even start: this is from the Kernel API Manual, section 9, and it's literally about catching people who write to your variable without permission. slay. let's get into it 🎀

✨ NAME ✨

ASSERT_EXCLUSIVE_WRITER — assert no concurrent writes to @var

like, be so fr, only ONE of you is allowed to write here bestie 🫶

🌸 SYNOPSIS 🌸

ASSERT_EXCLUSIVE_WRITER (var );

yes that's the whole vibe. one argument. iconic. 💅

🦋 ARGUMENTS 🦋

  • var — the variable to assert on. that's it. that's the girl. the one we're guarding with our whole life 🌈

💖 DESCRIPTION 💖

So basically this macro asserts that there are no concurrent writes to var — like, at all. Other readers though? totally allowed, they're welcome here, we love readers 🫶

The whole point is that this assertion can be used to specify properties of concurrent code where a violation can't be detected as a normal data race. lowkey a menace, highkey useful.

For example — picture this: we only have a single writer, but multiple concurrent readers. To avoid data races, all of these accesses have to be marked. But here's the thing, and it's giving plot twist: even concurrent marked writes racing with the single writer are bugs. 😭 And unfortunately, because they're marked, they're no longer data races. so the detector's just sitting there like 👁️👄👁️

For cases like these, we can use the macro as follows:

void writer(void) {
        spin_lock(update_foo_lock);
        ASSERT_EXCLUSIVE_WRITER(shared_foo);
        WRITE_ONCE(shared_foo, ...);
        spin_unlock(update_foo_lock);
}
void reader(void) {
        // update_foo_lock does not need to be held!
        ... = READ_ONCE(shared_foo);
}

see how the reader literally does not need to hold update_foo_lock? yeah. she's free. she's unbothered. she's reading. 🧚

⭐ NOTE ⭐

ASSERT_EXCLUSIVE_WRITER_SCOPED, if applicable, performs more thorough checking — but only if there's a clear scope where no concurrent writes are expected. so if you've got that scope, she's the upgrade bestie, no cap 🔥

👑 SEE ALSO 👑

All the related girls in the kernel file ./include/linux/kcsan-checks.h — go say hi 💜

and that's the whole page babes 💅 this one lives in ./include/linux and it's dated October 2026 — periodt, go assert those exclusive writers, ily, mwah 🍓✨

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