[ltt-dev] [PATCH 10/10] call_rcu: avoid useless futex wakeups

[Paolo Bonzini]

If the call_rcu thread is waiting for entries to accumulate, it is
not necessary to wake it up.  Use the BUSY flag to detect this.

Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
---
 urcu-call-rcu-impl.h |    8 +++++---
 1 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/urcu-call-rcu-impl.h b/urcu-call-rcu-impl.h
index d8570e3..165956e 100644
--- a/urcu-call-rcu-impl.h
+++ b/urcu-call-rcu-impl.h
@@ -514,10 +514,12 @@ void call_rcu(struct rcu_head *head,
 	head->func = func;
 	crdp = get_call_rcu_data();
 	cds_wfq_enqueue(&crdp->cbs, &head->next);
-	/* Write list before writing the flags.  */
+	/* Write list before checking/writing the flags.  */
 	cmm_smp_mb();
-	uatomic_or(&crdp->flags, URCU_CALL_RCU_BUSY);
-	wake_call_rcu_thread(crdp);
+	if ((uatomic_read(&crdp->flags) & URCU_CALL_RCU_BUSY) == 0) {
+		uatomic_or(&crdp->flags, URCU_CALL_RCU_BUSY);
+		wake_call_rcu_thread(crdp);
+	}
 }
 
 /*
-- 
1.7.4.4