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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 21:50:20 -0400 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Fri, Jul 26, 2013 at 09:13:32AM +0800, Fam Zheng wrote:
> 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.
OK, I can go with that. I can't think off the top of my head where
this would cause a problem if it is just used in place of current
bdrv_delete(), so long as everyone refs it when they should.
But then going with a bdrv_delete() equivalency model, I don't know if
it is appropriate to call bdrv_unref() in bdrv_detach_dev() (in patch
5/8). Maybe all that is really needed there is some more cleanup in
the places that call bdrv_detach_dev(), and then it would be OK.
>
> > 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
[Qemu-devel] [PATCH 8/8] nbd: use BlockDriverState refcnt, Fam Zheng, 2013/07/25