Re: [Qemu-devel] [PATCH v11 02/19] block: Pause all jobs during bdrv_reopen_multiple()

[Paolo Bonzini]

On 14/10/2016 15:08, Alberto Garcia wrote:
> When a BlockDriverState is about to be reopened it can trigger certain
> operations that need to write to disk. During this process a different
> block job can be woken up. If that block job completes and also needs
> to call bdrv_reopen() it can happen that it needs to do it on the same
> BlockDriverState that is still in the process of being reopened.
> 
> This can have fatal consequences, like in this example:
> 
>   1) Block job A starts and sleeps after a while.
>   2) Block job B starts and tries to reopen node1 (a qcow2 file).
>   3) Reopening node1 means flushing and replacing its qcow2 cache.
>   4) While the qcow2 cache is being flushed, job A wakes up.
>   5) Job A completes and reopens node1, replacing its cache.
>   6) Job B resumes, but the cache that was being flushed no longer
>      exists.
> 
> This patch splits the bdrv_drain_all() call to keep all block jobs
> paused during bdrv_reopen_multiple(), so that step 4 can never happen
> and the operation is safe.
> 
> Note that this scenario can only happen if both bdrv_reopen() calls
> are made by block jobs on the same backing chain. Otherwise there's no
> chance that the same BlockDriverState appears in both reopen queues.
> 
> Signed-off-by: Alberto Garcia <address@hidden>
> ---
>  block.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/block.c b/block.c
> index 7f3e7bc..adbecd0 100644
> --- a/block.c
> +++ b/block.c
> @@ -2090,7 +2090,7 @@ int bdrv_reopen_multiple(BlockReopenQueue *bs_queue, 
> Error **errp)
>  
>      assert(bs_queue != NULL);
>  
> -    bdrv_drain_all();
> +    bdrv_drain_all_begin();
>  
>      QSIMPLEQ_FOREACH(bs_entry, bs_queue, entry) {
>          if (bdrv_reopen_prepare(&bs_entry->state, bs_queue, &local_err)) {
> @@ -2120,6 +2120,9 @@ cleanup:
>          g_free(bs_entry);
>      }
>      g_free(bs_queue);
> +
> +    bdrv_drain_all_end();
> +
>      return ret;
>  }
>  
> 

Reviewed-by: Paolo Bonzini <address@hidden>