migration: Add migration_rp_wait|kick()

It's just a simple wrapper for rp_sem on either wait() or kick(), make it
even clearer on how it is used.  Prepared to be used even for other things.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-ID: <20231004220240.167175-8-peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

migration/migration.c

@@ -1763,6 +1763,16 @@ static void mark_source_rp_bad(MigrationState *s)
     s->rp_state.error = true;
 }
 
+void migration_rp_wait(MigrationState *s)
+{
+    qemu_sem_wait(&s->rp_state.rp_sem);
+}
+
+void migration_rp_kick(MigrationState *s)
+{
+    qemu_sem_post(&s->rp_state.rp_sem);
+}
+
 static struct rp_cmd_args {
     ssize_t     len; /* -1 = variable */
     const char *name;
@@ -1835,7 +1845,7 @@ static int migrate_handle_rp_resume_ack(MigrationState *s, uint32_t value)
                       MIGRATION_STATUS_POSTCOPY_ACTIVE);
 
     /* Notify send thread that time to continue send pages */
-    qemu_sem_post(&s->rp_state.rp_sem);
+    migration_rp_kick(s);
 
     return 0;
 }
@@ -2464,7 +2474,7 @@ static int postcopy_resume_handshake(MigrationState *s)
     qemu_savevm_send_postcopy_resume(s->to_dst_file);
 
     while (s->state == MIGRATION_STATUS_POSTCOPY_RECOVER) {
-        qemu_sem_wait(&s->rp_state.rp_sem);
+        migration_rp_wait(s);
     }
 
     if (s->state == MIGRATION_STATUS_POSTCOPY_ACTIVE) {

migration/migration.h

@@ -316,6 +316,12 @@ struct MigrationState {
          * be cleared in the rp_thread!
          */
         bool          rp_thread_created;
+        /*
+         * Used to synchronize between migration main thread and return
+         * path thread.  The migration thread can wait() on this sem, while
+         * other threads (e.g., return path thread) can kick it using a
+         * post().
+         */
         QemuSemaphore rp_sem;
         /*
          * We post to this when we got one PONG from dest. So far it's an
@@ -527,4 +533,13 @@ void migration_populate_vfio_info(MigrationInfo *info);
 void migration_reset_vfio_bytes_transferred(void);
 void postcopy_temp_page_reset(PostcopyTmpPage *tmp_page);
 
+/* Migration thread waiting for return path thread. */
+void migration_rp_wait(MigrationState *s);
+/*
+ * Kick the migration thread waiting for return path messages.  NOTE: the
+ * name can be slightly confusing (when read as "kick the rp thread"), just
+ * to remember the target is always the migration thread.
+ */
+void migration_rp_kick(MigrationState *s);
+
 #endif

migration/ram.c

@@ -4141,7 +4141,7 @@ static int ram_dirty_bitmap_sync_all(MigrationState *s, RAMState *rs)
 
     /* Wait until all the ramblocks' dirty bitmap synced */
     while (qatomic_read(&rs->postcopy_bmap_sync_requested)) {
-        qemu_sem_wait(&s->rp_state.rp_sem);
+        migration_rp_wait(s);
     }
 
     trace_ram_dirty_bitmap_sync_complete();
@@ -4149,11 +4149,6 @@ static int ram_dirty_bitmap_sync_all(MigrationState *s, RAMState *rs)
     return 0;
 }
 
-static void ram_dirty_bitmap_reload_notify(MigrationState *s)
-{
-    qemu_sem_post(&s->rp_state.rp_sem);
-}
-
 /*
  * Read the received bitmap, revert it as the initial dirty bitmap.
  * This is only used when the postcopy migration is paused but wants
@@ -4237,10 +4232,13 @@ int ram_dirty_bitmap_reload(MigrationState *s, RAMBlock *block)
     qatomic_dec(&rs->postcopy_bmap_sync_requested);
 
     /*
-     * We succeeded to sync bitmap for current ramblock. If this is
+     * We succeeded to sync bitmap for current ramblock. Always kick the
-     * the last one to sync, we need to notify the main send thread.
+     * migration thread to check whether all requested bitmaps are
+     * reloaded.  NOTE: it's racy to only kick when requested==0, because
+     * we don't know whether the migration thread may still be increasing
+     * it.
      */
-    ram_dirty_bitmap_reload_notify(s);
+    migration_rp_kick(s);
 
     ret = 0;
 out: