rcu_barrier(9)

💖 rcu_barrier(9) — the Kernel API Manual glow-up 🎀

Okay bestie, so you're in the Kernel API Manual, section 9, and the star of the show is rcu_barrier. Let's get into it ✨

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🌸 NAME 🌸

rcu_barrier — wait until all in-flight call_rcu() callbacks complete. That's it, that's the whole vibe. 🫶

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🦋 SYNOPSIS 🦋

void rcu_barrier (void );

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🎀 ARGUMENTS 🎀

• void — no arguments. Not one. Zero. Empty set, bestie. 💅

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✨ DESCRIPTION ✨

Okay so real talk, this is the part where you think it's a full "wait for a whole RCU grace period" situation, and it's not. Note that this primitive does not necessarily wait for an RCU grace period to complete. Like, at all. 🙈

For example, if there are no RCU callbacks queued anywhere in the system, then rcu_barrier is within its rights to return immediately, without waiting for anything, much less an RCU grace period. Literally just yeets itself back to the caller. 😭 In fact, rcu_barrier will normally not result in any RCU grace periods beyond those that were already destined to be executed. So it's not out here creating extra grace periods for fun — it only piggybacks on the ones already on the calendar. 🗓️

Also, quick heads up: in kernels built with CONFIG_RCU_LAZY=y, this function also hurries all pending lazy RCU callbacks. So the lazy ones get a little nudge. Not lazy anymore. Periodt. 🔥

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👑 SEE ALSO 👑

• Kernel file ./kernel/rcu/tree.c
• rcu_softirq_qs(9)
• 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_throttled(9)

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And that's the whole page, bestie — courtesy of ./kernel/rcu, dated October 2026, filed under rcu_barrier(9). Go forth and barrier responsibly 🧚💜