Re: [Qemu-devel] [PATCH v2 for-2.11] block: Make bdrv_next() keep strong references

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Nov 10, 2017 at 06:25:45PM +0100, Max Reitz wrote:
> On one hand, it is a good idea for bdrv_next() to return a strong
> reference because ideally nearly every pointer should be refcounted.
> This fixes intermittent failure of iotest 194.
> 
> On the other, it is absolutely necessary for bdrv_next() itself to keep
> a strong reference to both the BB (in its first phase) and the BDS (at
> least in the second phase) because when called the next time, it will
> dereference those objects to get a link to the next one.  Therefore, it
> needs these objects to stay around until then.  Just storing the pointer
> to the next in the iterator is not really viable because that pointer
> might become invalid as well.
> 
> Both arguments taken together means we should probably just invoke
> bdrv_ref() and blk_ref() in bdrv_next().  This means we have to assert
> that bdrv_next() is always called from the main loop, but that was
> probably necessary already before this patch and judging from the
> callers, it also looks to actually be the case.
> 
> Keeping these strong references means however that callers need to give
> them up if they decide to abort the iteration early.  They can do so
> through the new bdrv_next_cleanup() function.
> 
> Suggested-by: Kevin Wolf <kwolf@redhat.com>
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
> v2: Instead of keeping the strong reference in bdrv_drain_all_*() only,
>     have them for all callers of bdrv_next() [Fam, Kevin]
>     (Completely different patch now, so no git-backport-diff included
>      here)
> ---
>  include/block/block.h |  1 +
>  block.c               |  3 +++
>  block/block-backend.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++--
>  block/snapshot.c      |  6 ++++++
>  migration/block.c     |  1 +
>  5 files changed, 57 insertions(+), 2 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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