Re: [Qemu-devel] [PATCH v10 2/3] quorum: implement bdrv_add_child() and bdrv_del_child()

[Max Reitz <mreitz@redhat.com>]

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On 16.02.2016 10:37, 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        | 122 +++++++++++++++++++++++++++++++++++++++++++++++++-
>  include/block/block.h |   4 ++
>  3 files changed, 128 insertions(+), 6 deletions(-)
> 
> diff --git a/block.c b/block.c
> index 08aa979..c3c9dc0 100644
> --- a/block.c
> +++ b/block.c
> @@ -1198,10 +1198,10 @@ static int bdrv_fill_options(QDict **options, const char *filename,
>      return 0;
>  }
>  
> -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 = g_new(BdrvChild, 1);
>      *child = (BdrvChild) {
> diff --git a/block/quorum.c b/block/quorum.c
> index a5ae4b8..e5a7e4f 100644
> --- a/block/quorum.c
> +++ b/block/quorum.c
> @@ -24,6 +24,7 @@
>  #include "qapi/qmp/qstring.h"
>  #include "qapi-event.h"
>  #include "crypto/hash.h"
> +#include "qemu/bitmap.h"
>  
>  #define HASH_LENGTH 32
>  
> @@ -81,6 +82,8 @@ typedef struct BDRVQuorumState {
>      bool rewrite_corrupted;/* true if the driver must rewrite-on-read corrupted
>                              * block if Quorum is reached.
>                              */
> +    unsigned long *index_bitmap;
> +    int bsize;
>  
>      QuorumReadPattern read_pattern;
>  } BDRVQuorumState;
> @@ -876,9 +879,9 @@ 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");

Side note: Actually, we could work with 0 children, too. Quorum would
then need to implement bdrv_is_inserted() and return false if there are
no children.

But that is something that can be implemented later on if the need arises.

>          ret = -EINVAL;
>          goto exit;
>      }
> @@ -927,6 +930,7 @@ static int quorum_open(BlockDriverState *bs, QDict *options, int flags,
>      /* allocate the children array */
>      s->children = g_new0(BdrvChild *, s->num_children);
>      opened = g_new0(bool, s->num_children);
> +    s->index_bitmap = bitmap_new(s->num_children);
>  
>      for (i = 0; i < s->num_children; i++) {
>          char indexstr[32];
> @@ -942,6 +946,8 @@ static int quorum_open(BlockDriverState *bs, QDict *options, int flags,
>  
>          opened[i] = true;
>      }
> +    bitmap_set(s->index_bitmap, 0, s->num_children);
> +    s->bsize = s->num_children;
>  
>      g_free(opened);
>      goto exit;
> @@ -998,6 +1004,115 @@ static void quorum_attach_aio_context(BlockDriverState *bs,
>      }
>  }
>  
> +static int get_new_child_index(BDRVQuorumState *s)
> +{
> +    int index;
> +
> +    index = find_next_zero_bit(s->index_bitmap, s->bsize, 0);
> +    if (index < s->bsize) {
> +        return index;
> +    }
> +
> +    if ((s->bsize % BITS_PER_LONG) == 0) {
> +        s->index_bitmap = bitmap_zero_extend(s->index_bitmap, s->bsize,
> +                                             s->bsize + 1);

I think this function needs to be called unconditionally. Looking into
its implementation, its call to g_realloc() will not do anything (and it
will probably be pretty quick at that), but the following bitmap_clear()
will only clear the bits from old_nbits (s->bsize) to new_nbits
(s->bsize + 1).

Thus, if you only call this function every 32nd/64th child, only that
child's bit will be initialized to zero. All the rest is undefined.

You probably didn't notice because bitmap_new() returns a
zero-initialized bitmap, and thus you'd have to create around 64
children (on an x64 machine) to notice.

> +    }
> +
> +    return s->bsize++;
> +}
> +
> +static void remove_child_index(BDRVQuorumState *s, int index)
> +{
> +    int last_index;
> +    long new_len;

size_t would be the more appropriate type.

> +
> +    assert(index < s->bsize);
> +
> +    clear_bit(index, s->index_bitmap);
> +    if (index < s->bsize - 1) {
> +        /*
> +         * The last bit is always set, and we don't clear

s/don't/didn't/

> +         * the last bit.
> +         */
> +        return;
> +    }
> +
> +    last_index = find_last_bit(s->index_bitmap, s->bsize);

An assert(last_index < s->bsize); here wouldn't hurt.

(last_index == s->bsize would be the case if no bit is set in
s->index_bitmap anymore, which should be impossible.)

> +    s->bsize = last_index + 1;
> +    if (BITS_TO_LONGS(last_index + 1) == BITS_TO_LONGS(s->bsize)) {
> +        return;
> +    }
> +
> +    new_len = BITS_TO_LONGS(last_index + 1) * sizeof(unsigned long);

s/last_index + 1/s->bsize/ looks better to me.

> +    s->index_bitmap = g_realloc(s->index_bitmap, new_len);
> +}
> +
> +static void quorum_add_child(BlockDriverState *bs, BlockDriverState *child_bs,
> +                             Error **errp)
> +{
> +    BDRVQuorumState *s = bs->opaque;
> +    BdrvChild *child;
> +    char indexstr[32];
> +    int index, ret;
> +
> +    index = get_new_child_index(s);
> +    ret = snprintf(indexstr, 32, "children.%d", index);
> +    if (ret < 0 || ret >= 32) {
> +        error_setg(errp, "cannot generate child name");
> +        return;
> +    }
> +
> +    bdrv_drain(bs);
> +
> +    assert(s->num_children <= INT_MAX / sizeof(BdrvChild *));
> +    if (s->num_children == INT_MAX / sizeof(BdrvChild *)) {
> +        error_setg(errp, "Too many children");
> +        return;
> +    }
> +    s->children = g_renew(BdrvChild *, s->children, s->num_children + 1);
> +
> +    bdrv_ref(child_bs);
> +    child = bdrv_attach_child(bs, child_bs, indexstr, &child_format);
> +    s->children[s->num_children++] = child;
> +    set_bit(index, s->index_bitmap);
> +}
> +
> +static void quorum_del_child(BlockDriverState *bs, BlockDriverState *child_bs,
> +                             Error **errp)
> +{
> +    BDRVQuorumState *s = bs->opaque;
> +    BdrvChild *child;
> +    int i, index;
> +
> +    for (i = 0; i < s->num_children; i++) {
> +        if (s->children[i]->bs == child_bs) {
> +            break;
> +        }
> +    }
> +
> +    /* we have checked it in bdrv_del_child() */
> +    assert(i < s->num_children);
> +    child = s->children[i];
> +
> +    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" */

Optional: assert(!strncmp(child->name, "children.", 9));

> +    index = atoi(child->name + 9);

Optional: Assert absence of an error:

unsigned long index;
char *endptr;

index = strtoul(child->name + 9, &endptr, 10);
assert(index >= 0 && !*endptr);

Max

> +
> +    bdrv_drain(bs);
> +    /* We can safely remove this child now */
> +    memmove(&s->children[i], &s->children[i + 1],
> +            (s->num_children - i - 1) * sizeof(void *));
> +    s->children = g_renew(BdrvChild *, s->children, --s->num_children);
> +    remove_child_index(s, index);
> +    bdrv_unref_child(bs, child);
> +}
> +
>  static void quorum_refresh_filename(BlockDriverState *bs, QDict *options)
>  {
>      BDRVQuorumState *s = bs->opaque;
> @@ -1053,6 +1168,9 @@ 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 ecde190..4b787d2 100644
> --- a/include/block/block.h
> +++ b/include/block/block.h
> @@ -517,6 +517,10 @@ 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);
> 



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