Dead lock found in django transaction with select_for_update function

I have two functions: one for updating data and one for reading data. When they run concurrently in two threads, a deadlock occurs. Does anyone know why? The database's isolation level is set to REPEATABLE-READ. sleep function is used to increase the likelihood of a deadlock occurring.

@transaction.atomic()
def func_change_data():
    print("start")
    time.sleep(10)
    boos = Boo.objects.select_for_update(skip_locked=True).filter(pk=xxx)
    if not changes.exists():
        return

    print("sleep")
    time.sleep(10)
    boo = boos.first()
    boo.stage = 'stage_2'
    boo.save()
    print("end")

@transaction.atomic()
def func__read():
    boo = Boo.objects.select_for_update().get(
        pk=xxx,
        target_branch='master',
        stage='stage_1'
    )
    time.sleep(10)
    print("end sleep")

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