From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:41840) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1V2WbE-0004fR-NA for qemu-devel@nongnu.org; Thu, 25 Jul 2013 21:13:44 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1V2Wb9-0002zw-Do for qemu-devel@nongnu.org; Thu, 25 Jul 2013 21:13:40 -0400 Received: from mx1.redhat.com ([209.132.183.28]:7677) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1V2Wb9-0002zp-6R for qemu-devel@nongnu.org; Thu, 25 Jul 2013 21:13:35 -0400 Received: from int-mx11.intmail.prod.int.phx2.redhat.com (int-mx11.intmail.prod.int.phx2.redhat.com [10.5.11.24]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id r6Q1DYCN009870 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK) for ; Thu, 25 Jul 2013 21:13:34 -0400 Date: Fri, 26 Jul 2013 09:13:32 +0800 From: Fam Zheng Message-ID: <20130726011332.GC15504@T430s.redhat.com> References: <1374742906-4489-1-git-send-email-famz@redhat.com> <1374742906-4489-4-git-send-email-famz@redhat.com> <20130725131517.GB19811@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130725131517.GB19811@localhost.localdomain> Subject: Re: [Qemu-devel] [PATCH 3/8] block: implement reference count for BlockDriverState Reply-To: famz@redhat.com List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jeff Cody Cc: kwolf@redhat.com, pbonzini@redhat.com, qemu-devel@nongnu.org, stefanha@redhat.com 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 > > --- > > 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