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Date: Fri, 9 Apr 2010 19:17:45 +0100
Message-ID: <r2rfbd9d3991004091117m9d01fc59labc788ca5acd4b3a@mail.gmail.com>
Subject: Re: [Qemu-devel] [PATCH 2/2] block: Convert bdrv_first to QTAILQ
From: Stefan Hajnoczi <stefanha@gmail.com>
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To: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, qemu-devel@nongnu.org

On Fri, Apr 9, 2010 at 5:47 PM, Kevin Wolf <kwolf@redhat.com> wrote:
>> @@ -545,13 +542,15 @@ void bdrv_close(BlockDriverState *bs)
>>
>> =A0void bdrv_delete(BlockDriverState *bs)
>> =A0{
>> - =A0 =A0BlockDriverState **pbs;
>> + =A0 =A0BlockDriverState *bs1;
>>
>> - =A0 =A0pbs =3D &bdrv_first;
>> - =A0 =A0while (*pbs !=3D bs && *pbs !=3D NULL)
>> - =A0 =A0 =A0 =A0pbs =3D &(*pbs)->next;
>> - =A0 =A0if (*pbs =3D=3D bs)
>> - =A0 =A0 =A0 =A0*pbs =3D bs->next;
>> + =A0 =A0/* remove from list, if necessary */
>> + =A0 =A0QTAILQ_FOREACH(bs1, &bdrv_states, list) {
>> + =A0 =A0 =A0 =A0if (bs1 =3D=3D bs) {
>> + =A0 =A0 =A0 =A0 =A0 =A0QTAILQ_REMOVE(&bdrv_states, bs, list);
>> + =A0 =A0 =A0 =A0 =A0 =A0break;
>> + =A0 =A0 =A0 =A0}
>> + =A0 =A0}
>
> This loop looks strange, what is it used for? We had only a next pointer
> previously, so we needed to search the element. A QTAILQ has both prev
> and next pointers though, so you should be able to directly use
> QTAILQ_REMOVE.

Only named BlockDriverStates are added to the tail queue.  Those with
an empty string as their name will not be on the tail queue.

The QTAILQ_REMOVE macro will not work if the element isn't on the tail
queue.  I didn't see a clean way to check if an element is on a tail
queue using qemu-queue.h, so I kept the search behavior.  The check I
want is tge_prev !=3D NULL, I think.

I see three options:
1. Leave the search.
2. Modify qemu-queue.h to add a QTAILQ_ON_LIST(elm) macro.
3. Break the QTAILQ abstraction and test tge_prev directly.

What do you think?

Stefan