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Re: [Qemu-devel] [PATCH 3/8] block: implement reference count for BlockD
From: |
Jeff Cody |
Subject: |
Re: [Qemu-devel] [PATCH 3/8] block: implement reference count for BlockDriverState |
Date: |
Thu, 25 Jul 2013 09:15:17 -0400 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Thu, Jul 25, 2013 at 05:01:41PM +0800, Fam Zheng wrote:
> Introduce bdrv_ref/bdrv_unref to manage the lifecycle of
> BlockDriverState. They are unused for now but will used to replace
> bdrv_delete() later.
>
> Signed-off-by: Fam Zheng <address@hidden>
> ---
> block.c | 22 ++++++++++++++++++++++
> include/block/block.h | 2 ++
> include/block/block_int.h | 1 +
> 3 files changed, 25 insertions(+)
>
> diff --git a/block.c b/block.c
> index 6cd39fa..6f7ad7f 100644
> --- a/block.c
> +++ b/block.c
> @@ -306,6 +306,7 @@ BlockDriverState *bdrv_new(const char *device_name)
> bdrv_iostatus_disable(bs);
> notifier_list_init(&bs->close_notifiers);
> notifier_with_return_list_init(&bs->before_write_notifiers);
> + bs->refcnt = 1;
>
> return bs;
> }
> @@ -1511,6 +1512,9 @@ static void bdrv_move_feature_fields(BlockDriverState
> *bs_dest,
> /* dirty bitmap */
> bs_dest->dirty_bitmap = bs_src->dirty_bitmap;
>
> + /* reference count */
> + bs_dest->refcnt = bs_src->refcnt;
> +
> /* job */
> bs_dest->in_use = bs_src->in_use;
> bs_dest->job = bs_src->job;
> @@ -4385,6 +4389,24 @@ int64_t bdrv_get_dirty_count(BlockDriverState *bs)
> }
> }
>
> +/* Get a reference to bs */
> +void bdrv_ref(BlockDriverState *bs)
> +{
> + bs->refcnt++;
> +}
> +
> +/* Release a previously grabbed reference to bs.
> + * If after releasing, reference count is zero, the BlockDriverState is
> + * deleted. */
> +void bdrv_unref(BlockDriverState *bs)
> +{
> + assert(bs->refcnt > 0);
> + if (--bs->refcnt == 0) {
> + bdrv_close(bs);
> + bdrv_delete(bs);
> + }
> +}
The problem with this is that a caller to bdrv_unref() has no
way of knowing after calling bdrv_unref() if bs is still valid. We
can't just set bs to NULL after calling bdrv_unref() as with
bdrv_delete(), because now it may not have been freed.
Maybe bdrv_unref should either return the current bs pointer, or
alternatively accept as its argument a pointer to the BDS pointer:
void bdrv_unref(BlockDriverState **bs)
{
assert(*bs->refcnt > 0);
if (--*bs->refcnt == 0) {
bdrv_close(*bs);
bdrv_delete(*bs);
*bs = NULL;
}
}
Of course, all callers would need to then check for NULL.
Also, do we need to call bdrv_close() in here? In bdrv_delete(),
bdrv_close() is called prior to the free.
> +
> void bdrv_set_in_use(BlockDriverState *bs, int in_use)
> {
> assert(bs->in_use != in_use);
> diff --git a/include/block/block.h b/include/block/block.h
> index 742fce5..b33ef62 100644
> --- a/include/block/block.h
> +++ b/include/block/block.h
> @@ -356,6 +356,8 @@ int64_t bdrv_get_dirty_count(BlockDriverState *bs);
> void bdrv_enable_copy_on_read(BlockDriverState *bs);
> void bdrv_disable_copy_on_read(BlockDriverState *bs);
>
> +void bdrv_ref(BlockDriverState *bs);
> +void bdrv_unref(BlockDriverState *bs);
> void bdrv_set_in_use(BlockDriverState *bs, int in_use);
> int bdrv_in_use(BlockDriverState *bs);
>
> diff --git a/include/block/block_int.h b/include/block/block_int.h
> index c6ac871..a282d56 100644
> --- a/include/block/block_int.h
> +++ b/include/block/block_int.h
> @@ -294,6 +294,7 @@ struct BlockDriverState {
> BlockDeviceIoStatus iostatus;
> char device_name[32];
> HBitmap *dirty_bitmap;
> + int refcnt;
> int in_use; /* users other than guest access, eg. block migration */
> QTAILQ_ENTRY(BlockDriverState) list;
>
> --
> 1.8.3.2
>
>
[Qemu-devel] [PATCH 2/8] iscsi: use bdrv_new() instead of stack structure, Fam Zheng, 2013/07/25
[Qemu-devel] [PATCH 4/8] block: make bdrv_delete() static, Fam Zheng, 2013/07/25
[Qemu-devel] [PATCH 5/8] block: use BlockDriverState refcnt for device attach/detach, Fam Zheng, 2013/07/25
[Qemu-devel] [PATCH 6/8] migration: omit drive ref as we have bdrv_ref now, Fam Zheng, 2013/07/25
[Qemu-devel] [PATCH 7/8] xen_disk: simplify blk_disconnect with refcnt, Fam Zheng, 2013/07/25