migration: Yield bitmap_mutex properly when sending/sleeping

Don't take the bitmap mutex when sending pages, or when being throttled by
migration_rate_limit() (which is a bit tricky to call it here in ram code,
but seems still helpful).

It prepares for the possibility of concurrently sending pages in >1 threads
using the function ram_save_host_page() because all threads may need the
bitmap_mutex to operate on bitmaps, so that either sendmsg() or any kind of
qemu_sem_wait() blocking for one thread will not block the other from
progressing.

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

migration/ram.c

@@ -2480,16 +2480,22 @@ static void postcopy_preempt_reset_channel(RAMState *rs)
  * a host page in which case the remainder of the hostpage is sent.
  * Only dirty target pages are sent. Note that the host page size may
  * be a huge page for this block.
+ *
  * The saving stops at the boundary of the used_length of the block
  * if the RAMBlock isn't a multiple of the host page size.
  *
+ * The caller must be with ram_state.bitmap_mutex held to call this
+ * function.  Note that this function can temporarily release the lock, but
+ * when the function is returned it'll make sure the lock is still held.
+ *
  * Returns the number of pages written or negative on error
  *
  * @rs: current RAM state
  * @pss: data about the page we want to send
  */
 static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss)
 {
+    bool page_dirty, preempt_active = postcopy_preempt_active();
     int tmppages, pages = 0;
     size_t pagesize_bits =
         qemu_ram_pagesize(pss->block) >> TARGET_PAGE_BITS;
@@ -2513,22 +2519,40 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss)
             break;
         }
 
-        /* Check the pages is dirty and if it is send it */
-        if (migration_bitmap_clear_dirty(rs, pss->block, pss->page)) {
-            tmppages = ram_save_target_page(rs, pss);
-            if (tmppages < 0) {
-                return tmppages;
-            }
+        page_dirty = migration_bitmap_clear_dirty(rs, pss->block, pss->page);
 
-            pages += tmppages;
+        /* Check the pages is dirty and if it is send it */
+        if (page_dirty) {
             /*
-             * Allow rate limiting to happen in the middle of huge pages if
-             * something is sent in the current iteration.
+             * Properly yield the lock only in postcopy preempt mode
+             * because both migration thread and rp-return thread can
+             * operate on the bitmaps.
              */
-            if (pagesize_bits > 1 && tmppages > 0) {
-                migration_rate_limit();
+            if (preempt_active) {
+                qemu_mutex_unlock(&rs->bitmap_mutex);
+            }
+            tmppages = ram_save_target_page(rs, pss);
+            if (tmppages >= 0) {
+                pages += tmppages;
+                /*
+                 * Allow rate limiting to happen in the middle of huge pages if
+                 * something is sent in the current iteration.
+                 */
+                if (pagesize_bits > 1 && tmppages > 0) {
+                    migration_rate_limit();
+                }
+            }
+            if (preempt_active) {
+                qemu_mutex_lock(&rs->bitmap_mutex);
             }
+        } else {
+            tmppages = 0;
+        }
+
+        if (tmppages < 0) {
+            return tmppages;
         }
+
         pss->page = migration_bitmap_find_dirty(rs, pss->block, pss->page);
     } while ((pss->page < hostpage_boundary) &&
              offset_in_ramblock(pss->block,