Re: [Qemu-devel] [Qemu-block] [PATCH] block: check BlockDriverState object before dereference

[Paolo Bonzini]

On 21/07/2017 17:47, Stefan Hajnoczi wrote:
> Hmm...BlockDriverState still has bdrv_is_inserted() even though
> BlockBackend->root can be NULL?  CCing Markus in case he has thoughts on
> the BB/BDS split.
> 
> I find it weird that block-backend.c calls bdrv_inc_in_flight() and then
> bdrv_co_flush() will call it again.  Perhaps we need to do this so that
> blk_drain() works correctly but it's odd that they share the same
> counter variable.

See here:
https://lists.gnu.org/archive/html/qemu-devel/2016-10/msg02305.html

> I need to count requests at the BB level because the blk_aio_*
> operations have a separate bottom half that is invoked if either 1) they
> never reach BDS (because of an error); or 2) the bdrv_co_* call
> completes without yielding.  The count must be >0 when blk_aio_*
> returns, or bdrv_drain (and thus blk_drain) won't loop.  Because
> bdrv_drain and blk_drain are conflated, the counter must be the BDS one.
> 
> In turn, the BDS counter is needed because of the lack of isolated
> sections.  The right design would be for blk_isolate_begin to call
> blk_drain on *other* BlockBackends reachable in a child-to-parent visit.
>  Instead, until that is implemented, we have bdrv_drained_begin that
> emulates that through the same-named callback, followed by a
> parent-to-child bdrv_drain that is almost always unnecessary.

The full explanation of the long-term plans and what "isolated section"
means is at
https://lists.gnu.org/archive/html/qemu-devel/2016-10/msg02016.html.
(Unfortunately, since then we've reintroduced the "double aio_poll" hack
in BDRV_POLL_WHILE...).

Paolo

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