Re: [Qemu-devel] [PATCH v5 09/42] block: Include filters when freezing backing chain

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

13.06.2019 1:09, Max Reitz wrote:
> In order to make filters work in backing chains, the associated
> functions must be able to deal with them and freeze all filter links, be
> they COW or R/W filter links.
> 
> While at it, add some comments that note which functions require their
> caller to ensure that a given child link is not frozen, and how the
> callers do so.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>   block.c | 45 ++++++++++++++++++++++++++++++++-------------
>   1 file changed, 32 insertions(+), 13 deletions(-)
> 
> diff --git a/block.c b/block.c
> index 8438b0699e..45882a3470 100644
> --- a/block.c
> +++ b/block.c
> @@ -2214,12 +2214,15 @@ static void bdrv_replace_child_noperm(BdrvChild *child,
>    * If @new_bs is not NULL, bdrv_check_perm() must be called beforehand, as this
>    * function uses bdrv_set_perm() to update the permissions according to the new
>    * reference that @new_bs gets.
> + *
> + * Callers must ensure that child->frozen is false.
>    */
>   static void bdrv_replace_child(BdrvChild *child, BlockDriverState *new_bs)
>   {
>       BlockDriverState *old_bs = child->bs;
>       uint64_t perm, shared_perm;
>   
> +    /* Asserts that child->frozen == false */
>       bdrv_replace_child_noperm(child, new_bs);
>   
>       if (old_bs) {
> @@ -2360,6 +2363,7 @@ static void bdrv_detach_child(BdrvChild *child)
>       g_free(child);
>   }
>   
> +/* Callers must ensure that child->frozen is false. */

Is such a comment better than one-line extra assertion at start of the function body?

>   void bdrv_root_unref_child(BdrvChild *child)
>   {
>       BlockDriverState *child_bs;
> @@ -2369,6 +2373,7 @@ void bdrv_root_unref_child(BdrvChild *child)
>       bdrv_unref(child_bs);
>   }
>   
> +/* Callers must ensure that child->frozen is false. */
>   void bdrv_unref_child(BlockDriverState *parent, BdrvChild *child)
>   {
>       if (child == NULL) {
> @@ -2435,6 +2440,7 @@ void bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd,
>       }
>   
>       if (bs->backing) {
> +        /* Cannot be frozen, we checked that above */
>           bdrv_unref_child(bs, bs->backing);
>       }
>   
> @@ -3908,6 +3914,7 @@ static void bdrv_close(BlockDriverState *bs)
>   
>       if (bs->drv) {
>           if (bs->drv->bdrv_close) {
> +            /* Must unfreeze all children, so bdrv_unref_child() works */
>               bs->drv->bdrv_close(bs);
>           }
>           bs->drv = NULL;
> @@ -4281,17 +4288,20 @@ BlockDriverState *bdrv_find_base(BlockDriverState *bs)
>    * Return true if at least one of the backing links between @bs and
>    * @base is frozen. @errp is set if that's the case.
>    * @base must be reachable from @bs, or NULL.
> + * (Filters are treated as normal elements of the backing chain.)
>    */
>   bool bdrv_is_backing_chain_frozen(BlockDriverState *bs, BlockDriverState *base,
>                                     Error **errp)
>   {
>       BlockDriverState *i;
> +    BdrvChild *child;
>   
> -    for (i = bs; i != base; i = backing_bs(i)) {
> -        if (i->backing && i->backing->frozen) {
> +    for (i = bs; i != base; i = child_bs(child)) {
> +        child = bdrv_filtered_child(i);
> +
> +        if (child && child->frozen) {
>               error_setg(errp, "Cannot change '%s' link from '%s' to '%s'",
> -                       i->backing->name, i->node_name,
> -                       backing_bs(i)->node_name);
> +                       child->name, i->node_name, child->bs->node_name);
>               return true;
>           }
>       }
> @@ -4305,19 +4315,22 @@ bool bdrv_is_backing_chain_frozen(BlockDriverState *bs, BlockDriverState *base,
>    * none of the links are modified.
>    * @base must be reachable from @bs, or NULL.
>    * Returns 0 on success. On failure returns < 0 and sets @errp.
> + * (Filters are treated as normal elements of the backing chain.)
>    */
>   int bdrv_freeze_backing_chain(BlockDriverState *bs, BlockDriverState *base,
>                                 Error **errp)
>   {
>       BlockDriverState *i;
> +    BdrvChild *child;
>   
>       if (bdrv_is_backing_chain_frozen(bs, base, errp)) {
>           return -EPERM;
>       }
>   
> -    for (i = bs; i != base; i = backing_bs(i)) {
> -        if (i->backing) {
> -            i->backing->frozen = true;
> +    for (i = bs; i != base; i = child_bs(child)) {
> +        child = bdrv_filtered_child(i);
> +        if (child) {
> +            child->frozen = true;
>           }
>       }
>   
> @@ -4328,15 +4341,18 @@ int bdrv_freeze_backing_chain(BlockDriverState *bs, BlockDriverState *base,
>    * Unfreeze all backing links between @bs and @base. The caller must
>    * ensure that all links are frozen before using this function.
>    * @base must be reachable from @bs, or NULL.
> + * (Filters are treated as normal elements of the backing chain.)
>    */
>   void bdrv_unfreeze_backing_chain(BlockDriverState *bs, BlockDriverState *base)
>   {
>       BlockDriverState *i;
> +    BdrvChild *child;
>   
> -    for (i = bs; i != base; i = backing_bs(i)) {
> -        if (i->backing) {
> -            assert(i->backing->frozen);
> -            i->backing->frozen = false;
> +    for (i = bs; i != base; i = child_bs(child)) {
> +        child = bdrv_filtered_child(i);
> +        if (child) {
> +            assert(child->frozen);
> +            child->frozen = false;
>           }
>       }
>   }
> @@ -4438,8 +4454,11 @@ int bdrv_drop_intermediate(BlockDriverState *top, BlockDriverState *base,
>               }
>           }
>   
> -        /* Do the actual switch in the in-memory graph.
> -         * Completes bdrv_check_update_perm() transaction internally. */
> +        /*
> +         * Do the actual switch in the in-memory graph.
> +         * Completes bdrv_check_update_perm() transaction internally.
> +         * c->frozen is false, we have checked that above.
> +         */
>           bdrv_ref(base);
>           bdrv_replace_child(c, base);
>           bdrv_unref(top);
> 

Hmm, OK, it's better than it was, so:
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

But I have one thought: we check that frozen is false at some point, and then
do some logic around this child. Where is guarantee, that someone else will not
set frozen=true during some yield, for example? So, do we need a kind of child_mutex,
or something like this?

-- 
Best regards,
Vladimir