migration/multifd: Protect accesses to migration_threads

This doubly linked list is common for all the multifd and migration
threads so we need to avoid concurrent access.

Add a mutex to protect the data from concurrent access. This fixes a
crash when removing two MigrationThread objects from the list at the
same time during cleanup of multifd threads.

Fixes: 6713262 ("migration: Introduce interface query-migrationthreads")
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20230607161306.31425-3-farosas@suse.de>
Signed-off-by: Juan Quintela <quintela@redhat.com>

migration/threadinfo.c

@@ -10,23 +10,35 @@
  *  See the COPYING file in the top-level directory.
  */
 
+#include "qemu/osdep.h"
+#include "qemu/queue.h"
+#include "qemu/lockable.h"
 #include "threadinfo.h"
 
+QemuMutex migration_threads_lock;
 static QLIST_HEAD(, MigrationThread) migration_threads;
 
+static void __attribute__((constructor)) migration_threads_init(void)
+{
+    qemu_mutex_init(&migration_threads_lock);
+}
+
 MigrationThread *migration_threads_add(const char *name, int thread_id)
 {
     MigrationThread *thread =  g_new0(MigrationThread, 1);
     thread->name = name;
     thread->thread_id = thread_id;
 
-    QLIST_INSERT_HEAD(&migration_threads, thread, node);
+    WITH_QEMU_LOCK_GUARD(&migration_threads_lock) {
+        QLIST_INSERT_HEAD(&migration_threads, thread, node);
+    }
 
     return thread;
 }
 
 void migration_threads_remove(MigrationThread *thread)
 {
+    QEMU_LOCK_GUARD(&migration_threads_lock);
     if (thread) {
         QLIST_REMOVE(thread, node);
         g_free(thread);
@@ -39,6 +51,7 @@ MigrationThreadInfoList *qmp_query_migrationthreads(Error **errp)
     MigrationThreadInfoList **tail = &head;
     MigrationThread *thread = NULL;
 
+    QEMU_LOCK_GUARD(&migration_threads_lock);
     QLIST_FOREACH(thread, &migration_threads, node) {
         MigrationThreadInfo *info = g_new0(MigrationThreadInfo, 1);
         info->name = g_strdup(thread->name);

migration/threadinfo.h

@@ -10,8 +10,6 @@
  *  See the COPYING file in the top-level directory.
  */
 
-#include "qemu/queue.h"
-#include "qemu/osdep.h"
 #include "qapi/error.h"
 #include "qapi/qapi-commands-migration.h"