[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] [PATCH 3/8] block: implement reference count for BlockD
From: |
Fam Zheng |
Subject: |
Re: [Qemu-devel] [PATCH 3/8] block: implement reference count for BlockDriverState |
Date: |
Fri, 26 Jul 2013 09:13:32 +0800 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Thu, 07/25 09:15, Jeff Cody wrote:
> 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.
>
By calling bdrv_unref, it means the caller is not going to use bs any
more. In other words, bdrv_unref() is a bdrv_delete() as seen by the
caller, if bs is still valid pointer after unref, it's no longer safe
for the caller: it can be freed by other code, in any time, but the
caller can't know.
> 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.
>
Yes, it can be omited.
> > +
> > 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
> >
> >
--
Fam
[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