void pdu_free(V9fsPDU *pdu)
{
    V9fsState *s = pdu->s;

    g_assert(!pdu->cancelled);
    QLIST_REMOVE(pdu, next);
    QLIST_INSERT_HEAD(&s->free_list, pdu, next);
}

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.