Re: [Qemu-devel] [PATCH v2] blockjob: update nodes head while removing all bdrv

[Max Reitz <mreitz@redhat.com>]

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On 10.09.19 15:36, Sergio Lopez wrote:
> block_job_remove_all_bdrv() iterates through job->nodes, calling
> bdrv_root_unref_child() for each entry. The call to the latter may
> reach child_job_[can_]set_aio_ctx(), which will also attempt to
> traverse job->nodes, potentially finding entries that where freed
> on previous iterations.
> 
> To avoid this situation, update job->nodes head on each iteration to
> ensure that already freed entries are no longer linked to the list.
> 
> RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1746631
> Signed-off-by: Sergio Lopez <slp@redhat.com>
> ---
> Changelog
> 
> v2:
>  - Avoid leaking job->nodes (thanks Max Reitz)
> ---
>  blockjob.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> ---
> diff --git a/blockjob.c b/blockjob.c
> index 6e32d1a0c0..ffda6dd1e4 100644
> --- a/blockjob.c
> +++ b/blockjob.c
> @@ -187,13 +187,21 @@ static const BdrvChildRole child_job = {
>  
>  void block_job_remove_all_bdrv(BlockJob *job)
>  {
> -    GSList *l;
> +    GSList *l, *orig_nodes;
> +
> +    orig_nodes = job->nodes;
>      for (l = job->nodes; l; l = l->next) {
>          BdrvChild *c = l->data;
>          bdrv_op_unblock_all(c->bs, job->blocker);
>          bdrv_root_unref_child(c);
> +        /*
> +         * The call above may reach child_job_[can_]set_aio_ctx(), which will
> +         * also traverse job->nodes, so update the head here to make sure it
> +         * doesn't attempt to process an already freed BdrvChild.
> +         */
> +        job->nodes = l->next;
>      }
> -    g_slist_free(job->nodes);
> +    g_slist_free(orig_nodes);
>      job->nodes = NULL;

Hm, this assignment is now a no-op.

I think I’d just rewrite the whole function in the following fashion:

orig_nodes = job->nodes;
while (job->nodes) {
    BdrvChild *c = job->nodes->data;
    [...]
    job->nodes = job->nodes->next;
}
g_slist_free(orig_nodes);

What do you think?

>  }
>  
> 



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