RCU_LOCKDEP_WARN(9)

📦 version 7.3.1, in nixpkgs unstable

💅 RCU_LOCKDEP_WARN — bestie edition

okay so this is the kernel API manual page for RCU_LOCKDEP_WARN, and yes it's a manpage, and yes it's giving "9" (kernel section, iykyk). Sit down, grab a strawberry drink 🍓, we're gonna get through this together.

✨ NAME ✨

RCU_LOCKDEP_WARN — the little gremlin that goes and emits a lockdep splat if the specified condition is met 💀

Like. It's a warning-dispenser. You hand it a condition, and if that condition is true, it yells at you through lockdep. That's the whole vibe.

🦋 SYNOPSIS 🦋

RCU_LOCKDEP_WARN (c , s );

that's it. two args. two tiny little args. and yet. 😭

🎀 ARGUMENTS 🎀

  • c — the condition you want checked. this is the thing that gets evaluated. if it comes back true, bestie, we have opinions.
  • s — the informative message. this is what gets said when the splat happens, so make it actually informative, okay? it's your one chance to be helpful 🙈

🌸 DESCRIPTION 🌸

Okay so here's the actual lore, and it's lowkey kind of genius. ✨

Before it even looks at (c), this thing checks debug_lockdep_rcu_enabled. Why? Because during early boot, lockdep hasn't been initialized yet, and if you just went straight in and checked (c) you'd get splats that are fully not the vibe — noisy, pointless, embarrassing. So it does the vibe check first. 🫶

Then it checks (c).

And then — and this is the part that genuinely ate — it rechecks debug_lockdep_rcu_enabled after checking (c). Because there's a race window where lockdep could get disabled in between, and without that second check you'd get false-positive splats. Not me getting blamed for a splat caused by a race 😭💀

See commit 3066820034b5dd ("rcu: Reject RCU_LOCKDEP_WARN false positives") for more detail — like, the literal receipts. go read it. it's iconic.

ngl the whole design is just: check, look, check again. three-step skincare routine for splats. 🧴

👑 SEE ALSO 👑

The kernel file ./include/linux/rcupdate.h has the whole friend group. Here's the entire crew, no cap, all of them:

thirty-five of them. thirty-five!! the way this function has more friends than me 😭💜

anyway bestie, that's the whole page. go emit your splats responsibly. 🎀

source: ./include/linux · dated October 2026 · section RCU_LOCKDEP_WARN(9) — mwah, love u, bye 💋✨
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