[Qemu-devel] [RFC 6/6] memory: make cpu_physical_memory_sync_dirty_bitmap() fully atomic

[Stefan Hajnoczi <stefanha@redhat.com>]

The fast path of cpu_physical_memory_sync_dirty_bitmap() directly
manipulates the dirty bitmap.  Use atomic_xchg() to make the
test-and-clear atomic.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 include/exec/ram_addr.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
index cdcbe9a..3d2a4c1 100644
--- a/include/exec/ram_addr.h
+++ b/include/exec/ram_addr.h
@@ -195,13 +195,13 @@ uint64_t cpu_physical_memory_sync_dirty_bitmap(unsigned long *dest,
         unsigned long *src = ram_list.dirty_memory[DIRTY_MEMORY_MIGRATION];
 
         for (k = page; k < page + nr; k++) {
-            if (src[k]) {
+            unsigned long bits = atomic_xchg(&src[k], 0);
+            if (bits) {
                 unsigned long new_dirty;
                 new_dirty = ~dest[k];
-                dest[k] |= src[k];
-                new_dirty &= src[k];
+                dest[k] |= bits;
+                new_dirty &= bits;
                 num_dirty += ctpopl(new_dirty);
-                src[k] = 0;
             }
         }
     } else {
-- 
2.1.0