Re: [Qemu-devel] [RFC] potential risk for macro QTAILQ_INSERT_BEFORE

[Jan Kiszka]

On 2011-09-28 05:38, Wayne Xia wrote:
>     Hi, during my coding, I found macro a bit different from other
> QTAIL macros.
> 
> QTAILQ_INSERT_AFTER was defined as:
> -----------------------------------------------------------
> #define QTAILQ_INSERT_AFTER(head, listelm, elm, field) do {             \
>         if (((elm)->field.tqe_next = (listelm)->field.tqe_next) != NULL)\
>                 (elm)->field.tqe_next->field.tqe_prev =                 \
>                     &(elm)->field.tqe_next;                             \
>         else                                                            \
>                 (head)->tqh_last = &(elm)->field.tqe_next;              \
>         (listelm)->field.tqe_next = (elm);                              \
>         (elm)->field.tqe_prev = &(listelm)->field.tqe_next;             \
> } while (/*CONSTCOND*/0)
> ---------------------------------------------------------
>  QTAILQ_INSERT_BEFORE is defined as following:
> 
> #define QTAILQ_INSERT_BEFORE(listelm, elm, field) do {
>         (elm)->field.tqe_prev = (listelm)->field.tqe_prev;              \
>         (elm)->field.tqe_next = (listelm);                              \
>         *(listelm)->field.tqe_prev = (elm);                             \
>         (listelm)->field.tqe_prev = &(elm)->field.tqe_next;             \
> } while (/*CONSTCOND*/0)
> ----------------------------------------------------------
> 
>     It did not take care of "head" as QTAILQ_INSERT_AFTER did, so I am
> wondering what would happen if I use QTAILQ_INSERT_BEFORE to insert one
> element to a queue that have only one element in it, would it happen
> that the queue head pointer is not updated and the real first element
> is lost? Currently some codes in qemu have used this macro.

The code is fine: a QTAILQ head consists of a dummy entry element that
looks for the first element as if a normal element would precede it. In
contrast, there is no dummy "end element", the last one just points to
NULL. Therefore we need to handle this separately.

Jan

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