rcu_softirq_qs(9)

💅 rcu_softirq_qs(9) — from the Kernel API Manual, bestie 🎀

okay so picture this: you're deep in softirq land, and RCU is out here waiting on you like 👀. This little function is your "hey bestie, I'm not holding anything, you can go" moment. Let's get into it ✨

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🌸 NAME 🌸

rcu_softirq_qs — gives you a whole little set of RCU quiescent states right inside softirq processing 🦋

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✨ SYNOPSIS ✨

void rcu_softirq_qs (void );

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🎀 ARGUMENTS 🎀

• void — no arguments. zero. nada. she takes nothing and gives you quiescent states. iconic behávior honestly 👑

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💖 DESCRIPTION 💖

So what she does: she marks a quiescent state for RCU, Tasks RCU, and Tasks Trace RCU. all three. at once. ate and left no crumbs 💅

But listen — this is a special-purpose function, okay? It's meant for the softirq infrastructure and maybe, like, the occasional long-running softirq handler. It's not a casual "let me just sprinkle this everywhere" situation, iykyk 🙈

ngl the key thing to internalize: from RCU's point of view, calling rcu_softirq_qs is equivalent to momentarily completely enabling preemption. Like... for one hot second, RCU sees everything wide open. main character energy 🌈

For example, say you've got this code:

local_bh_disable;
do_something;
rcu_softirq_qs;  // A
do_something_else;
local_bh_enable;  // B

A call to synchronize_rcu that started concurrently with the call to do_something is guaranteed to wait only until execution reached statement A. ⭐

But without that rcu_softirq_qs? Then that same synchronize_rcu would instead be guaranteed to wait all the way until execution reached statement B. 😭 so yeah, that one call is doing a LOT of heavy lifting. lowkey the whole plot twist 🔥

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💀 SEE ALSO 💀

Kernel file ./kernel/rcu/tree.c and her whole friend group 🫶:

• rcu_watching_snap_stopped_since(9)
• rcu_is_cpu_rrupt_from_idle(9)
• rcu_irq_exit_check_preempt(9)
• __rcu_irq_enter_check_tick(9)
• rcu_is_watching(9)
• rcu_set_gpwrap_lag(9)
• call_rcu_hurry(9)
• call_rcu(9)
• synchronize_rcu(9)
• get_completed_synchronize_rcu_full(9)
• get_state_synchronize_rcu(9)
• get_state_synchronize_rcu_full(9)
• start_poll_synchronize_rcu(9)
• start_poll_synchronize_rcu_full(9)
• poll_state_synchronize_rcu(9)
• poll_state_synchronize_rcu_full(9)
• cond_synchronize_rcu(9)
• cond_synchronize_rcu_full(9)
• rcu_barrier(9)
• rcu_barrier_throttled(9)

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and that's the whole page, bestie!! 💜 this one comes to you from ./kernel/rcu, dated October 2026 — no cap, that's the source, the version and the date all in one cute little sign-off 🍓✨ go forth and mark those quiescent states, you've got this 🧚‍♀️