dump-guest-memory: Block live migration

Both dump-guest-memory and live migration caches vm state at the beginning.
Either of them entering the other one will cause race on the vm state, and even
more severe on that (please refer to the crash report in the bug link).

Let's block live migration in dump-guest-memory, and that'll also block
dump-guest-memory if it detected that we're during a live migration.

Side note: migrate_del_blocker() can be called even if the blocker is not
inserted yet, so it's safe to unconditionally delete that blocker in
dump_cleanup (g_slist_remove allows no-entry-found case).

Suggested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1996609
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

dump/dump.c

@@ -29,6 +29,7 @@
 #include "qemu/error-report.h"
 #include "qemu/main-loop.h"
 #include "hw/misc/vmcoreinfo.h"
+#include "migration/blocker.h"
 
 #ifdef TARGET_X86_64
 #include "win_dump.h"
@@ -47,6 +48,8 @@
 
 #define MAX_GUEST_NOTE_SIZE (1 << 20) /* 1MB should be enough */
 
+static Error *dump_migration_blocker;
+
 #define ELF_NOTE_SIZE(hdr_size, name_size, desc_size)   \
     ((DIV_ROUND_UP((hdr_size), 4) +                     \
       DIV_ROUND_UP((name_size), 4) +                    \
@@ -101,6 +104,7 @@ static int dump_cleanup(DumpState *s)
             qemu_mutex_unlock_iothread();
         }
     }
+    migrate_del_blocker(dump_migration_blocker);
 
     return 0;
 }
@@ -2005,6 +2009,21 @@ void qmp_dump_guest_memory(bool paging, const char *file,
         return;
     }
 
+    if (!dump_migration_blocker) {
+        error_setg(&dump_migration_blocker,
+                   "Live migration disabled: dump-guest-memory in progress");
+    }
+
+    /*
+     * Allows even for -only-migratable, but forbid migration during the
+     * process of dump guest memory.
+     */
+    if (migrate_add_blocker_internal(dump_migration_blocker, errp)) {
+        /* Remember to release the fd before passing it over to dump state */
+        close(fd);
+        return;
+    }
+
     s = &dump_state_global;
     dump_state_prepare(s);