From 4b9432ed65cb3a692e8d8bd86abb93f5f2dc5b69 Mon Sep 17 00:00:00 2001 From: Frederic Weisbecker Date: Tue, 29 Apr 2025 15:43:01 +0200 Subject: [PATCH] rcu/exp: Remove confusing needless full barrier on task unblock A full memory barrier in the RCU-PREEMPT task unblock path advertizes to order the context switch (or rather the accesses prior to rcu_read_unlock()) with the expedited grace period fastpath. However the grace period can not complete without the rnp calling into rcu_report_exp_rnp() with the node locked. This reports the quiescent state in a fully ordered fashion against updater's accesses thanks to: 1) The READ-SIDE smp_mb__after_unlock_lock() barrier across nodes locking while propagating QS up to the root. 2) The UPDATE-SIDE smp_mb__after_unlock_lock() barrier while holding the the root rnp to wait/check for the GP completion. 3) The (perhaps redundant given step 1) and 2)) smp_mb() in rcu_seq_end() before the grace period completes. This makes the explicit barrier in this place superfluous. Therefore remove it as it is confusing. Acked-by: Paul E. McKenney Signed-off-by: Frederic Weisbecker Signed-off-by: Joel Fernandes Signed-off-by: Neeraj Upadhyay (AMD) --- kernel/rcu/tree_plugin.h | 1 - 1 file changed, 1 deletion(-) diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 0b0f56f6abc85..0532a13cb75ea 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -534,7 +534,6 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags) WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq && (!empty_norm || rnp->qsmask)); empty_exp = sync_rcu_exp_done(rnp); - smp_mb(); /* ensure expedited fastpath sees end of RCU c-s. */ np = rcu_next_node_entry(t, rnp); list_del_init(&t->rcu_node_entry); t->rcu_blocked_node = NULL; -- 2.47.2