Re: [Qemu-devel] [PATCH 1/2] commit: reopen overlay_bs before base

[Max Reitz <mreitz@redhat.com>]

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On 28.10.2015 14:43, Alberto Garcia wrote:
> 'block-commit' needs write access to two different nodes of the chain:
> 
> - 'base', because that's where the data is written to.
> - the overlay of 'top', because it needs to update the backing file
>   string to point to 'base' after the operation.
> 
> Both images have to be opened in read-write mode, and commit_start()
> takes care of reopening them if necessary.
> 
> With the current implementation, however, when overlay_bs is reopened
> in read-write mode it has the side effect of making 'base' read-only
> again, eventually making 'block-commit' fail.
> 
> This needs to be fixed in bdrv_reopen(), but until we get to that it
> can be worked around simply by swapping the order of base and
> overlay_bs in the reopen queue.
> 
> In order to reproduce this bug, overlay_bs needs to be initially in
> read-only mode. That is: the 'top' parameter of 'block-commit' cannot
> be the active layer nor its immediate backing chain.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> ---
>  block/commit.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)

Reviewed-by: Max Reitz <mreitz@redhat.com>


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