[Qemu-devel] [PATCH 2/2] [RFC] Convert Clock Timerlists to RCU V3

[Mike Day <ncmike@ncultra.org>]

timerlists is a list of lists that holds active timers, among other
items. It is read-mostly. This patch converts read access to the
timerlists to use RCU.

Rather than introduce a second mutex for timerlists, which would
require nested mutexes to in order to write to the timerlists, use one
QemuMutex in the QemuClock structure for all write access to any list
hanging off the QemuClock structure.

This patch applies against Paolo Bonzini's rcu branch:
https://github.com/bonzini/qemu/tree/rcu and also requires the
previously submitted patch ae11e1c "Convert active timers list to use
RCU for read operations V3."

V3:
- timerlists modifications split to a separate patch (this one).
- Addressed comments from Alex Bligh and Paolo Bonzini.

Signed-off-by: Mike Day <ncmike@ncultra.org>
---
 qemu-timer.c | 31 ++++++++++++++++++++++++++-----
 1 file changed, 26 insertions(+), 5 deletions(-)

diff --git a/qemu-timer.c b/qemu-timer.c
index 57a1545..4144e54 100644
--- a/qemu-timer.c
+++ b/qemu-timer.c
@@ -74,6 +74,7 @@ struct QEMUTimerList {
     QEMUTimerListNotifyCB *notify_cb;
     void *notify_opaque;
     QemuEvent timers_done_ev;
+    struct rcu_head rcu;
 };
 
 /**
@@ -111,6 +112,13 @@ QEMUTimerList *timerlist_new(QEMUClockType type,
     return timer_list;
 }
 
+static void reclaim_timerlist(struct rcu_head *rcu)
+{
+    QEMUTimerList *tl = container_of(rcu, QEMUTimerList, rcu);
+    g_free(tl);
+}
+
+
 void timerlist_free(QEMUTimerList *timer_list)
 {
     assert(!timerlist_has_timers(timer_list));
@@ -118,7 +126,7 @@ void timerlist_free(QEMUTimerList *timer_list)
         QLIST_REMOVE(timer_list, list);
     }
     qemu_event_destroy(&timer_list->timers_done_ev);
-    g_free(timer_list);
+    call_rcu1(&timer_list->rcu, reclaim_timerlist);
 }
 
 static void qemu_clock_init(QEMUClockType type)
@@ -143,9 +151,11 @@ void qemu_clock_notify(QEMUClockType type)
 {
     QEMUTimerList *timer_list;
     QEMUClock *clock = qemu_clock_ptr(type);
-    QLIST_FOREACH(timer_list, &clock->timerlists, list) {
+    rcu_read_lock();
+    QLIST_FOREACH_RCU(timer_list, &clock->timerlists, list) {
         timerlist_notify(timer_list);
     }
+    rcu_read_unlock();
 }
 
 void qemu_clock_enable(QEMUClockType type, bool enabled)
@@ -157,9 +167,11 @@ void qemu_clock_enable(QEMUClockType type, bool enabled)
     if (enabled && !old) {
         qemu_clock_notify(type);
     } else if (!enabled && old) {
-        QLIST_FOREACH(tl, &clock->timerlists, list) {
+        rcu_read_lock();
+        QLIST_FOREACH_RCU(tl, &clock->timerlists, list) {
             qemu_event_wait(&tl->timers_done_ev);
         }
+        rcu_read_unlock();
     }
 }
 
@@ -243,10 +255,12 @@ int64_t qemu_clock_deadline_ns_all(QEMUClockType type)
     int64_t deadline = -1;
     QEMUTimerList *timer_list;
     QEMUClock *clock = qemu_clock_ptr(type);
-    QLIST_FOREACH(timer_list, &clock->timerlists, list) {
+    rcu_read_lock();
+    QLIST_FOREACH_RCU(timer_list, &clock->timerlists, list) {
         deadline = qemu_soonest_timeout(deadline,
                                         timerlist_deadline_ns(timer_list));
     }
+    rcu_read_unlock();
     return deadline;
 }
 
@@ -262,11 +276,13 @@ QEMUTimerList *qemu_clock_get_main_loop_timerlist(QEMUClockType type)
 
 void timerlist_notify(QEMUTimerList *timer_list)
 {
-    if (timer_list->notify_cb) {
+    rcu_read_lock();
+    if (atomic_rcu_read(&timer_list->notify_cb)) {
         timer_list->notify_cb(timer_list->notify_opaque);
     } else {
         qemu_notify_event();
     }
+    rcu_read_unlock();
 }
 
 /* Transition function to convert a nanosecond timeout to ms
@@ -585,13 +601,18 @@ void qemu_clock_register_reset_notifier(QEMUClockType type,
                                         Notifier *notifier)
 {
     QEMUClock *clock = qemu_clock_ptr(type);
+    qemu_mutex_lock(&clock->timer_lock);
     notifier_list_add(&clock->reset_notifiers, notifier);
+    qemu_mutex_unlock(&clock->timer_lock);
 }
 
 void qemu_clock_unregister_reset_notifier(QEMUClockType type,
                                           Notifier *notifier)
 {
+    QEMUClock *clock = qemu_clock_ptr(type);
+    qemu_mutex_lock(&clock->timer_lock);
     notifier_remove(notifier);
+    qemu_mutex_unlock(&clock->timer_lock);
 }
 
 void init_clocks(void)
-- 
1.9.0