[ltt-dev] Userspace RCU git fixes

[Paolo Bonzini]

> Just to let you know that I pushed two updates into urcu: one fixes a
> grace period hang caused by a missing wakeup in the synchronize_rcu QSBR
> code. This appears to hit us due to the more fine-grained wakeup
> code brought by Paolo. The wakeup was really missing from the
> synchronize_rcu code (so Paolo's code just triggered an existing
> problem). I thought it would be good to let you know the effect: grace
> periods are delayed forever. This problem never appeared in a release (I
> caught it before).

Good catch.  Why not use rcu_thread_offline/online in synchronize_rcu,
instead of touching rcu_reader.ctr directly?  I had this in my QEMU
branch but hadn't posted yet because it was meant as a cleanup only.

Paolo

----------------------- 8< ---------------------

>From 7ad6897f696034ef0651c912e43931a2b0bbe631 Mon Sep 17 00:00:00 2001
From: Paolo Bonzini <pbonzini at redhat.com>
Date: Mon, 12 Sep 2011 09:24:18 +0200
Subject: [PATCH] urcu-qsbr: use rcu_thread_offline/rcu_thread_online instead of inlining them

---
 urcu-qsbr.c |   40 +++++++++++++++++-----------------------
 1 files changed, 17 insertions(+), 23 deletions(-)

diff --git a/urcu-qsbr.c b/urcu-qsbr.c
index 1dc9979..1adaa94 100644
--- a/urcu-qsbr.c
+++ b/urcu-qsbr.c
@@ -208,21 +208,17 @@ void synchronize_rcu(void)
 	was_online = rcu_reader.ctr;
 
 	/* All threads should read qparity before accessing data structure
-	 * where new ptr points to.
-	 */
-	/* Write new ptr before changing the qparity */
-	cmm_smp_mb();
-
-	/*
+	 * where new ptr points to.  In the "then" case, rcu_thread_offline
+	 * includes a memory barrier.
+	 *
 	 * Mark the writer thread offline to make sure we don't wait for
 	 * our own quiescent state. This allows using synchronize_rcu()
 	 * in threads registered as readers.
 	 */
-	if (was_online) {
-		CMM_STORE_SHARED(rcu_reader.ctr, 0);
-		cmm_smp_mb();	/* write rcu_reader.ctr before read futex */
-		wake_up_gp();
-	}
+	if (was_online)
+		rcu_thread_offline();
+	else
+		cmm_smp_mb();
 
 	mutex_lock(&rcu_gp_lock);
 
@@ -263,9 +259,9 @@ out:
 	 * freed.
 	 */
 	if (was_online)
-		_CMM_STORE_SHARED(rcu_reader.ctr,
-				  CMM_LOAD_SHARED(rcu_gp_ctr));
-	cmm_smp_mb();
+		rcu_thread_online();
+	else
+		cmm_smp_mb();
 }
 #else /* !(CAA_BITS_PER_LONG < 64) */
 void synchronize_rcu(void)
@@ -279,12 +275,10 @@ void synchronize_rcu(void)
 	 * our own quiescent state. This allows using synchronize_rcu()
 	 * in threads registered as readers.
 	 */
-	cmm_smp_mb();
-	if (was_online) {
-		CMM_STORE_SHARED(rcu_reader.ctr, 0);
-		cmm_smp_mb();	/* write rcu_reader.ctr before read futex */
-		wake_up_gp();
-	}
+	if (was_online)
+		rcu_thread_offline();
+	else
+		cmm_smp_mb();
 
 	mutex_lock(&rcu_gp_lock);
 	if (cds_list_empty(&registry))
@@ -294,9 +288,9 @@ out:
 	mutex_unlock(&rcu_gp_lock);
 
 	if (was_online)
-		_CMM_STORE_SHARED(rcu_reader.ctr,
-				  CMM_LOAD_SHARED(rcu_gp_ctr));
-	cmm_smp_mb();
+		rcu_thread_online();
+	else
+		cmm_smp_mb();
 }
 #endif  /* !(CAA_BITS_PER_LONG < 64) */
 
-- 
1.7.6