Re: [Qemu-devel] [PATCH v13 2/3] quorum: implement bdrv_add_child() and bdrv_del_child()

[Changlong Xie <xiecl.fnst@cn.fujitsu.com>]

On 05/06/2016 11:20 PM, Max Reitz wrote:
> On 13.04.2016 10:33, Changlong Xie wrote:
>> From: Wen Congyang <wency@cn.fujitsu.com>
>>
>> Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
>> Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
>> Signed-off-by: Gonglei <arei.gonglei@huawei.com>
>> Signed-off-by: Changlong Xie <xiecl.fnst@cn.fujitsu.com>
>> ---
>>   block.c               |  8 +++---
>>   block/quorum.c        | 78 +++++++++++++++++++++++++++++++++++++++++++++++++--
>>   include/block/block.h |  4 +++
>>   3 files changed, 84 insertions(+), 6 deletions(-)
>
> Design-wise: Nice change. It's a bit strange now to have gaps in the
> child naming, but this strategy is very simple to implement and the
> order of the children used for FIFO is the same as the numerical order
> of the indices in the child names, so I'm happy.

I think this is the simplest approach too : )

>
>> diff --git a/block.c b/block.c
>> index 68cd3b2..4bdc6b3 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -1176,10 +1176,10 @@ BdrvChild *bdrv_root_attach_child(BlockDriverState *child_bs,
>>       return child;
>>   }
>>
>> -static BdrvChild *bdrv_attach_child(BlockDriverState *parent_bs,
>> -                                    BlockDriverState *child_bs,
>> -                                    const char *child_name,
>> -                                    const BdrvChildRole *child_role)
>> +BdrvChild *bdrv_attach_child(BlockDriverState *parent_bs,
>> +                             BlockDriverState *child_bs,
>> +                             const char *child_name,
>> +                             const BdrvChildRole *child_role)
>>   {
>>       BdrvChild *child = bdrv_root_attach_child(child_bs, child_name, child_role);
>>       QLIST_INSERT_HEAD(&parent_bs->children, child, next);
>> diff --git a/block/quorum.c b/block/quorum.c
>> index da15465..2553f82 100644
>> --- a/block/quorum.c
>> +++ b/block/quorum.c
>> @@ -14,6 +14,7 @@
>>    */
>>
>>   #include "qemu/osdep.h"
>> +#include "qemu/cutils.h"
>>   #include "block/block_int.h"
>>   #include "qapi/qmp/qbool.h"
>>   #include "qapi/qmp/qdict.h"
>> @@ -67,6 +68,9 @@ typedef struct QuorumVotes {
>>   typedef struct BDRVQuorumState {
>>       BdrvChild **children;  /* children BlockDriverStates */
>>       int num_children;      /* children count */
>> +    uint64_t last_index;   /* indicate the child role name of the last
>> +                            * element of children array
>
> The name (and the comment) is a bit misleading, as it's not the index of
> the last child but one after that, i.e. the index of the next child
> should it be added.
>
> So maybe this variable should be called "next_child_index" or something
> similar, and the comment should reflect that.
>

Will fix.

>> +                            */
>>       int threshold;         /* if less than threshold children reads gave the
>>                               * same result a quorum error occurs.
>>                               */
>> @@ -898,9 +902,9 @@ static int quorum_open(BlockDriverState *bs, QDict *options, int flags,
>>           ret = -EINVAL;
>>           goto exit;
>>       }
>> -    if (s->num_children < 2) {
>> +    if (s->num_children < 1) {
>>           error_setg(&local_err,
>> -                   "Number of provided children must be greater than 1");
>> +                   "Number of provided children must be 1 or more");
>>           ret = -EINVAL;
>>           goto exit;
>>       }
>> @@ -964,6 +968,7 @@ static int quorum_open(BlockDriverState *bs, QDict *options, int flags,
>>
>>           opened[i] = true;
>>       }
>> +    s->last_index = s->num_children;
>>
>>       g_free(opened);
>>       goto exit;
>> @@ -1020,6 +1025,72 @@ static void quorum_attach_aio_context(BlockDriverState *bs,
>>       }
>>   }
>>
>> +static void quorum_add_child(BlockDriverState *bs, BlockDriverState *child_bs,
>> +                             Error **errp)
>> +{
>> +    BDRVQuorumState *s = bs->opaque;
>> +    BdrvChild *child;
>> +    char indexstr[32];
>> +    int ret;
>> +
>> +    assert(s->num_children <= INT_MAX / sizeof(BdrvChild *) &&
>> +           s->last_index <= UINT64_MAX);
>> +    if (s->num_children == INT_MAX / sizeof(BdrvChild *) ||
>> +        s->last_index == UINT64_MAX) {
>> +        error_setg(errp, "Too many children");
>> +        return;
>> +    }
>> +
>> +    ret = snprintf(indexstr, 32, "children.%" PRIu64, s->last_index);
>> +    if (ret < 0 || ret >= 32) {
>> +        error_setg(errp, "cannot generate child name");
>> +        return;
>> +    }
>> +    s->last_index++;
>> +
>> +    bdrv_drain(bs);
>
> We have bdrv_drained_begin() and bdrv_drained_end() now. Perhaps we
> should make use of them and call bdrv_drained_begin() here...

Ditto

>
>> +
>> +    /* We can safely add the child now */
>> +    bdrv_ref(child_bs);
>> +    child = bdrv_attach_child(bs, child_bs, indexstr, &child_format);
>> +    s->children = g_renew(BdrvChild *, s->children, s->num_children + 1);
>> +    s->children[s->num_children++] = child;
>
> ...and bdrv_drained_end() here.

Ditto

>
>> +}
>> +
>> +static void quorum_del_child(BlockDriverState *bs, BdrvChild *child,
>> +                             Error **errp)
>> +{
>> +    BDRVQuorumState *s = bs->opaque;
>> +    int i;
>> +
>> +    for (i = 0; i < s->num_children; i++) {
>> +        if (s->children[i] == child) {
>> +            break;
>> +        }
>> +    }
>> +
>> +    /* we have checked it in bdrv_del_child() */
>> +    assert(i < s->num_children);
>> +
>> +    if (s->num_children <= s->threshold) {
>> +        error_setg(errp,
>> +            "The number of children cannot be lower than the vote threshold %d",
>> +            s->threshold);
>> +        return;
>> +    }
>> +
>> +    /* child->name is "children.%d" */
>> +    assert(!strncmp(child->name, "children.", 9));
>
> This could be removed now, too. I was asking for these assertions (of
> which in this version only this one is left) because we were using the
> child name to infer the index in the child bitmap before.
>
> Now we're not using the child name here at all, so we can just drop this
> assertion.

Will remove it in next version.

>
>> +
>> +    bdrv_drain(bs);
>
> As above, bdrv_drained_begin() here...

Will fix it.

>
>> +
>> +    /* We can safely remove this child now */
>> +    memmove(&s->children[i], &s->children[i + 1],
>> +            (s->num_children - i - 1) * sizeof(BdrvChild *));
>> +    s->children = g_renew(BdrvChild *, s->children, --s->num_children);
>> +    bdrv_unref_child(bs, child);
>
> ...and bdrv_drained_end() here may be better than a plain bdrv_drain().
>

Ditto.

>> +}
>> +
>>   static void quorum_refresh_filename(BlockDriverState *bs, QDict *options)
>>   {
>>       BDRVQuorumState *s = bs->opaque;
>> @@ -1075,6 +1146,9 @@ static BlockDriver bdrv_quorum = {
>>       .bdrv_detach_aio_context            = quorum_detach_aio_context,
>>       .bdrv_attach_aio_context            = quorum_attach_aio_context,
>>
>> +    .bdrv_add_child                     = quorum_add_child,
>> +    .bdrv_del_child                     = quorum_del_child,
>> +
>>       .is_filter                          = true,
>>       .bdrv_recurse_is_first_non_filter   = quorum_recurse_is_first_non_filter,
>>   };
>> diff --git a/include/block/block.h b/include/block/block.h
>> index 694ca76..52902cd 100644
>> --- a/include/block/block.h
>> +++ b/include/block/block.h
>> @@ -476,6 +476,10 @@ void bdrv_disable_copy_on_read(BlockDriverState *bs);
>>   void bdrv_ref(BlockDriverState *bs);
>>   void bdrv_unref(BlockDriverState *bs);
>>   void bdrv_unref_child(BlockDriverState *parent, BdrvChild *child);
>> +BdrvChild *bdrv_attach_child(BlockDriverState *parent_bs,
>> +                             BlockDriverState *child_bs,
>> +                             const char *child_name,
>> +                             const BdrvChildRole *child_role);
>>
>>   bool bdrv_op_is_blocked(BlockDriverState *bs, BlockOpType op, Error **errp);
>>   void bdrv_op_block(BlockDriverState *bs, BlockOpType op, Error *reason);
>>
>
> None of the things I pointed out above is critical, but I'd still rather
> see them fixed before giving my R-b.

I'm glab to hear that, will send out next version.

Thanks
	-Xie
>
> Max
>