[Qemu-devel] [PATCH 7/9] rcu: prod call_rcu thread when calling synchronize_rcu

[Paolo Bonzini <pbonzini@redhat.com>]

call_rcu operates on the principle that either there is a steady stream of
incoming RCU callbacks, or it is not worthwhile to wake up and process the
few that are there.

This however makes it hard to assert in testcases that all RCU callbacks
are processed.  To avoid this, make call_rcu also process callbacks if there
is a steady stream of synchronize_rcu calls.

This avoids deadlocks in the upcoming test-rcu-list unit test, which waits
for call_rcu to reclaim all nodes that it allocates.  Especially with very
high load on the host, call_rcu decided to wait for a few more callbacks
to pile up, but the test was done and was not going to produce more.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 util/rcu.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/util/rcu.c b/util/rcu.c
index c9c3e6e..aa9f639 100644
--- a/util/rcu.c
+++ b/util/rcu.c
@@ -48,6 +48,9 @@ unsigned long rcu_gp_ctr = RCU_GP_LOCKED;
 QemuEvent rcu_gp_event;
 static QemuMutex rcu_gp_lock;
 
+static int rcu_call_count;
+static QemuEvent rcu_call_ready_event;
+
 /*
  * Check whether a quiescent state was crossed between the beginning of
  * update_counter_and_wait and now.
@@ -149,6 +152,9 @@ void synchronize_rcu(void)
     }
 
     qemu_mutex_unlock(&rcu_gp_lock);
+    if (atomic_read(&rcu_call_count)) {
+        qemu_event_set(&rcu_call_ready_event);
+    }
 }
 
 
@@ -159,8 +165,6 @@ void synchronize_rcu(void)
  */
 static struct rcu_head dummy;
 static struct rcu_head *head = &dummy, **tail = &dummy.next;
-static int rcu_call_count;
-static QemuEvent rcu_call_ready_event;
 
 static void enqueue(struct rcu_head *node)
 {
-- 
1.8.3.1