[Qemu-devel] [PATCH 06/15] memory: protect current_map by RCU

[Paolo Bonzini <pbonzini@redhat.com>]

Replace the flat_view_mutex by RCU, avoiding futex contention for
dataplane on large systems and many iothreads.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 include/exec/memory.h |  5 +++++
 memory.c              | 54 ++++++++++++++++++++++-----------------------------
 2 files changed, 28 insertions(+), 31 deletions(-)

diff --git a/include/exec/memory.h b/include/exec/memory.h
index 0cd96b1..06ffa1d 100644
--- a/include/exec/memory.h
+++ b/include/exec/memory.h
@@ -33,6 +33,7 @@
 #include "qemu/notify.h"
 #include "qapi/error.h"
 #include "qom/object.h"
+#include "qemu/rcu.h"
 
 #define MAX_PHYS_ADDR_SPACE_BITS 62
 #define MAX_PHYS_ADDR            (((hwaddr)1 << MAX_PHYS_ADDR_SPACE_BITS) - 1)
@@ -207,9 +208,13 @@ struct MemoryListener {
  */
 struct AddressSpace {
     /* All fields are private. */
+    struct rcu_head rcu;
     char *name;
     MemoryRegion *root;
+
+    /* Accessed via RCU.  */
     struct FlatView *current_map;
+
     int ioeventfd_nb;
     struct MemoryRegionIoeventfd *ioeventfds;
     struct AddressSpaceDispatch *dispatch;
diff --git a/memory.c b/memory.c
index 8c3d8c0..a844ced 100644
--- a/memory.c
+++ b/memory.c
@@ -33,26 +33,12 @@ static bool memory_region_update_pending;
 static bool ioeventfd_update_pending;
 static bool global_dirty_log = false;
 
-/* flat_view_mutex is taken around reading as->current_map; the critical
- * section is extremely short, so I'm using a single mutex for every AS.
- * We could also RCU for the read-side.
- *
- * The BQL is taken around transaction commits, hence both locks are taken
- * while writing to as->current_map (with the BQL taken outside).
- */
-static QemuMutex flat_view_mutex;
-
 static QTAILQ_HEAD(memory_listeners, MemoryListener) memory_listeners
     = QTAILQ_HEAD_INITIALIZER(memory_listeners);
 
 static QTAILQ_HEAD(, AddressSpace) address_spaces
     = QTAILQ_HEAD_INITIALIZER(address_spaces);
 
-static void memory_init(void)
-{
-    qemu_mutex_init(&flat_view_mutex);
-}
-
 typedef struct AddrRange AddrRange;
 
 /*
@@ -242,6 +228,7 @@ struct FlatRange {
  * order.
  */
 struct FlatView {
+    struct rcu_head rcu;
     unsigned ref;
     FlatRange *ranges;
     unsigned nr;
@@ -654,10 +641,10 @@ static FlatView *address_space_get_flatview(AddressSpace *as)
 {
     FlatView *view;
 
-    qemu_mutex_lock(&flat_view_mutex);
-    view = as->current_map;
+    rcu_read_lock();
+    view = atomic_rcu_read(&as->current_map);
     flatview_ref(view);
-    qemu_mutex_unlock(&flat_view_mutex);
+    rcu_read_unlock();
     return view;
 }
 
@@ -766,10 +753,9 @@ static void address_space_update_topology(AddressSpace *as)
     address_space_update_topology_pass(as, old_view, new_view, false);
     address_space_update_topology_pass(as, old_view, new_view, true);
 
-    qemu_mutex_lock(&flat_view_mutex);
-    flatview_unref(as->current_map);
-    as->current_map = new_view;
-    qemu_mutex_unlock(&flat_view_mutex);
+    /* Writes are protected by the BQL.  */
+    atomic_rcu_set(&as->current_map, new_view);
+    call_rcu(old_view, flatview_unref, rcu);
 
     /* Note that all the old MemoryRegions are still alive up to this
      * point.  This relieves most MemoryListeners from the need to
@@ -1957,10 +1943,6 @@ void memory_listener_unregister(MemoryListener *listener)
 
 void address_space_init(AddressSpace *as, MemoryRegion *root, const char *name)
 {
-    if (QTAILQ_EMPTY(&address_spaces)) {
-        memory_init();
-    }
-
     memory_region_transaction_begin();
     as->root = root;
     as->current_map = g_new(FlatView, 1);
@@ -1974,15 +1956,10 @@ void address_space_init(AddressSpace *as, MemoryRegion *root, const char *name)
     memory_region_transaction_commit();
 }
 
-void address_space_destroy(AddressSpace *as)
+static void do_address_space_destroy(AddressSpace *as)
 {
     MemoryListener *listener;
 
-    /* Flush out anything from MemoryListeners listening in on this */
-    memory_region_transaction_begin();
-    as->root = NULL;
-    memory_region_transaction_commit();
-    QTAILQ_REMOVE(&address_spaces, as, address_spaces_link);
     address_space_destroy_dispatch(as);
 
     QTAILQ_FOREACH(listener, &memory_listeners, link) {
@@ -1994,6 +1971,21 @@ void address_space_destroy(AddressSpace *as)
     g_free(as->ioeventfds);
 }
 
+void address_space_destroy(AddressSpace *as)
+{
+    /* Flush out anything from MemoryListeners listening in on this */
+    memory_region_transaction_begin();
+    as->root = NULL;
+    memory_region_transaction_commit();
+    QTAILQ_REMOVE(&address_spaces, as, address_spaces_link);
+
+    /* At this point, as->dispatch and as->current_map are dummy
+     * entries that the guest should never use.  Wait for the old
+     * values to expire before freeing the data.
+     */
+    call_rcu(as, do_address_space_destroy, rcu);
+}
+
 bool io_mem_read(MemoryRegion *mr, hwaddr addr, uint64_t *pval, unsigned size)
 {
     return memory_region_dispatch_read(mr, addr, pval, size);
-- 
1.8.3.1