Re: [Qemu-devel] [Qemu-block] [PATCH v2 00/16] block: Protect AIO context change with perm API

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Wed, Apr 19, 2017 at 05:43:40PM +0800, Fam Zheng wrote:
> v2: Address Stefan's comments:
> 
>     - Clean up redundancy in bdrv_format_default_perms change.
>     - Add a test case to check both success/failure cases.
>       A failure case is not possible at user interface level because of other
>       checks we have, so write a unit test in tests/test-blk-perm.c.
> 
> Eject / change of scsi-cd on a virtio-scsi dataplane bus causes abort() because
> the new BDS doesn't get proper bdrv_set_aio_context().
> 
> Store the AioContext in BB and do it in blk_insert_bs. That is done by
> Vladimir's patch.
> 
> Other patches are to make sure such a bdrv_set_aio_context() doesn't interfere
> with other BBs using other nodes from this graph.

Looks pretty good.  I had two comments that apply across all patches:

First, it is not safe to enable the new permission without registering
an aio notifier.  Another user could look up the BDS and call
bdrv_set_aio_context() on it.  I believe this bug is present for block
jobs that have additional BDSes like base/target/etc.

Second, patches that post-pone bdrv_set_aio_context() must take care to
acquire the AioContext for BDS accesses that happen before the next
bdrv_set_aio_context() call.

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