migration: Make from_dst_file accesses thread-safe

Accessing from_dst_file is potentially racy in current code base like below:

  if (s->from_dst_file)
    do_something(s->from_dst_file);

Because from_dst_file can be reset right after the check in another
thread (rp_thread).  One example is migrate_fd_cancel().

Use the same qemu_file_lock to protect it too, just like to_dst_file.

When it's safe to access without lock, comment it.

There's one special reference in migration_thread() that can be replaced by
the newly introduced rp_thread_created flag.

Reported-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Lukas Straub <lukasstraub2@web.de>
Message-Id: <20210722175841.938739-3-peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  with Peter's fixup

1 files changed, 5 insertions, 3 deletions

diff --git a/migration/migration.h b/migration/migration.h
index c302879fad..7a5aa8c2fd 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -154,12 +154,13 @@ struct MigrationState {
     QemuThread thread;
     QEMUBH *vm_start_bh;
     QEMUBH *cleanup_bh;
+    /* Protected by qemu_file_lock */
     QEMUFile *to_dst_file;
     QIOChannelBuffer *bioc;
     /*
-     * Protects to_dst_file pointer.  We need to make sure we won't
-     * yield or hang during the critical section, since this lock will
-     * be used in OOB command handler.
+     * Protects to_dst_file/from_dst_file pointers.  We need to make sure we
+     * won't yield or hang during the critical section, since this lock will be
+     * used in OOB command handler.
      */
     QemuMutex qemu_file_lock;
 
@@ -192,6 +193,7 @@ struct MigrationState {
 
     /* State related to return path */
     struct {
+        /* Protected by qemu_file_lock */
         QEMUFile     *from_dst_file;
         QemuThread    rp_thread;
         bool          error;