migration: Add postcopy_has_request()

Add a helper to detect whether postcopy has pending request.

Since at it, cleanup the code a bit, e.g. in unqueue_page() we shouldn't need
to check it again on queue empty because we're the only one (besides cleanup
code, which should never run during this process) that will take a request off
the list, so the request list can only grow but not shrink under the hood.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

migration/ram.c

@@ -355,6 +355,12 @@ static RAMState *ram_state;
 
 static NotifierWithReturnList precopy_notifier_list;
 
+/* Whether postcopy has queued requests? */
+static bool postcopy_has_request(RAMState *rs)
+{
+    return !QSIMPLEQ_EMPTY_ATOMIC(&rs->src_page_requests);
+}
+
 void precopy_infrastructure_init(void)
 {
     notifier_with_return_list_init(&precopy_notifier_list);
@@ -1539,28 +1545,33 @@ static bool find_dirty_block(RAMState *rs, PageSearchStatus *pss, bool *again)
  */
 static RAMBlock *unqueue_page(RAMState *rs, ram_addr_t *offset)
 {
+    struct RAMSrcPageRequest *entry;
     RAMBlock *block = NULL;
 
-    if (QSIMPLEQ_EMPTY_ATOMIC(&rs->src_page_requests)) {
+    if (!postcopy_has_request(rs)) {
         return NULL;
     }
 
     QEMU_LOCK_GUARD(&rs->src_page_req_mutex);
-    if (!QSIMPLEQ_EMPTY(&rs->src_page_requests)) {
-        struct RAMSrcPageRequest *entry =
-                                QSIMPLEQ_FIRST(&rs->src_page_requests);
-        block = entry->rb;
-        *offset = entry->offset;
-
-        if (entry->len > TARGET_PAGE_SIZE) {
-            entry->len -= TARGET_PAGE_SIZE;
-            entry->offset += TARGET_PAGE_SIZE;
-        } else {
-            memory_region_unref(block->mr);
-            QSIMPLEQ_REMOVE_HEAD(&rs->src_page_requests, next_req);
-            g_free(entry);
-            migration_consume_urgent_request();
-        }
+
+    /*
+     * This should _never_ change even after we take the lock, because no one
+     * should be taking anything off the request list other than us.
+     */
+    assert(postcopy_has_request(rs));
+
+    entry = QSIMPLEQ_FIRST(&rs->src_page_requests);
+    block = entry->rb;
+    *offset = entry->offset;
+
+    if (entry->len > TARGET_PAGE_SIZE) {
+        entry->len -= TARGET_PAGE_SIZE;
+        entry->offset += TARGET_PAGE_SIZE;
+    } else {
+        memory_region_unref(block->mr);
+        QSIMPLEQ_REMOVE_HEAD(&rs->src_page_requests, next_req);
+        g_free(entry);
+        migration_consume_urgent_request();
     }
 
     return block;
@@ -2992,7 +3003,7 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
         t0 = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
         i = 0;
         while ((ret = qemu_file_rate_limit(f)) == 0 ||
-                !QSIMPLEQ_EMPTY(&rs->src_page_requests)) {
+               postcopy_has_request(rs)) {
             int pages;
 
             if (qemu_file_get_error(f)) {