From 677ab23bdf416ec8f3ecaf10d7cc8d0ccb46adab Mon Sep 17 00:00:00 2001 From: Frederic Weisbecker Date: Wed, 15 May 2024 14:53:32 +0200 Subject: [PATCH] rcu/exp: Remove redundant full memory barrier at the end of GP A full memory barrier is necessary at the end of the expedited grace period to order: 1) The grace period completion (pictured by the GP sequence number) with all preceding accesses. This pairs with rcu_seq_end() performed by the concurrent kworker. 2) The grace period completion and subsequent post-GP update side accesses. Pairs again against rcu_seq_end(). This full barrier is already provided by the final sync_exp_work_done() test, making the subsequent explicit one redundant. Remove it and improve comments. Signed-off-by: Frederic Weisbecker Signed-off-by: Paul E. McKenney Reviewed-by: Boqun Feng Reviewed-by: Neeraj Upadhyay --- kernel/rcu/tree_exp.h | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h index 1dbad2442e8d..4acd29d16fdb 100644 --- a/kernel/rcu/tree_exp.h +++ b/kernel/rcu/tree_exp.h @@ -265,7 +265,12 @@ static bool sync_exp_work_done(unsigned long s) { if (rcu_exp_gp_seq_done(s)) { trace_rcu_exp_grace_period(rcu_state.name, s, TPS("done")); - smp_mb(); /* Ensure test happens before caller kfree(). */ + /* + * Order GP completion with preceding accesses. Order also GP + * completion with post GP update side accesses. Pairs with + * rcu_seq_end(). + */ + smp_mb(); return true; } return false; @@ -967,7 +972,6 @@ void synchronize_rcu_expedited(void) rnp = rcu_get_root(); wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3], sync_exp_work_done(s)); - smp_mb(); /* Work actions happen before return. */ /* Let the next expedited grace period start. */ mutex_unlock(&rcu_state.exp_mutex); -- 2.50.1