[2.4, 2.5, USB] locking issue

["Samium Gromoff" <_deepfire@mail.ru>]

        The possible problem is encountered in ehci-q.c and ehci-sched.c
  in 2.4.19-pre9 and in one occurence in ehci-q.c of 2.5.47.

        the offending pattern is the same in both files:

        if (!list_empty (qtd_list)) {
 -----------------------8<----------------------------------------------
                list_splice (qtd_list, &qh->qtd_list);
                qh_update (qh, list_entry (qtd_list->next, struct ehci_qtd, qtd\_list));
 -----------------------8<----------------------------------------------
        } else {
                qh->hw_qtd_next = qh->hw_alt_next = EHCI_LIST_END;
        }


        since list_splice() the qtd_list is diposed of its belongings and
        immediately in the next line we rely on qtd_list->next to point
        at an existing list_head.

        i haven`t noticed any locking out there, and i`m afraid of what
        could result from a preemption happening between these two lines.


regards, Samium Gromoff
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