Re: [Qemu-devel] [PATCH] rng: switch request queue to QSIMPLEQ

[Ladi Prosek]

On Fri, Mar 4, 2016 at 10:12 AM, Paolo Bonzini <address@hidden> wrote:
>
>
> On 04/03/2016 09:04, Ladi Prosek wrote:
>>>> >> +    QSIMPLEQ_INIT(&s->requests);
>>>> >>  }
>>> >
>>> > This init here isn't necessary, the accessors for the queue will take
>>> > care of this.
>> We are basically purging the queue here and we want to leave it in a
>> consistent state. Without the QSIMPLEQ_INIT the queue head would
>> become a pair of dangling pointers. Let me know if I misunderstood
>> your comment.
>
> It wouldn't, check out QSIMPLEQ_REMOVE_HEAD:
>
> #define QSIMPLEQ_REMOVE_HEAD(head, field) do {
>     if (((head)->sqh_first = (head)->sqh_first->field.sqe_next) == NULL)
>         (head)->sqh_last = &(head)->sqh_first;
> } while (/*CONSTCOND*/0)
>
> The queue would become { NULL, &s->requests.sqh_first }.  So the
> QSIMPLEQ_INIT is indeed redundant.

Right, but we're not running QSIMPLEQ_REMOVE_HEAD in this function. We
iterate the queue and free all elements without writing anything to
the head or to the next ptr. This is the only "write" we do in
rng_backend_free_requests.

> Paolo