Re: [PATCH 1/3] block: Make bdrv_refresh_limits() non-recursive

[Eric Blake <eblake@redhat.com>]

On Tue, Feb 15, 2022 at 02:57:25PM +0100, Hanna Reitz wrote:
> bdrv_refresh_limits() recurses down to the node's children.  That does
> not seem necessary: When we refresh limits on some node, and then
> recurse down and were to change one of its children's BlockLimits, then
> that would mean we noticed the changed limits by pure chance.  The fact
> that we refresh the parent's limits has nothing to do with it, so the
> reason for the change probably happened before this point in time, and
> we should have refreshed the limits then.
> 
> On the other hand, we do not have infrastructure for noticing that block
> limits change after they have been initialized for the first time (this
> would require propagating the change upwards to the respective node's
> parents), and so evidently we consider this case impossible.
> 
> If this case is impossible, then we will not need to recurse down in
> bdrv_refresh_limits().  Every node's limits are initialized in
> bdrv_open_driver(), and are refreshed whenever its children change.
> We want to use the childrens' limits to get some initial default, but we
> can just take them, we do not need to refresh them.
> 
> The problem with recursing is that bdrv_refresh_limits() is not atomic.
> It begins with zeroing BDS.bl, and only then sets proper, valid limits.
> If we do not drain all nodes whose limits are refreshed, then concurrent
> I/O requests can encounter invalid request_alignment values and crash
> qemu.  Therefore, a recursing bdrv_refresh_limits() requires the whole
> subtree to be drained, which is currently not ensured by most callers.
> 
> A non-recursive bdrv_refresh_limits() only requires the node in question
> to not receive I/O requests, and this is done by most callers in some
> way or another:
> - bdrv_open_driver() deals with a new node with no parents yet
> - bdrv_set_file_or_backing_noperm() acts on a drained node
> - bdrv_reopen_commit() acts only on drained nodes
> - bdrv_append() should in theory require the node to be drained; in
>   practice most callers just lock the AioContext, which should at least
>   be enough to prevent concurrent I/O requests from accessing invalid
>   limits
> 
> So we can resolve the bug by making bdrv_refresh_limits() non-recursive.

Long explanation, but very helpful.

> 
> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1879437
> Signed-off-by: Hanna Reitz <hreitz@redhat.com>
> ---
>  block/io.c | 4 ----
>  1 file changed, 4 deletions(-)

And deceptively simple fix!

Reviewed-by: Eric Blake <eblake@redhat.com>

> 
> diff --git a/block/io.c b/block/io.c
> index 4e4cb556c5..c3e7301613 100644
> --- a/block/io.c
> +++ b/block/io.c
> @@ -189,10 +189,6 @@ void bdrv_refresh_limits(BlockDriverState *bs, Transaction *tran, Error **errp)
>      QLIST_FOREACH(c, &bs->children, next) {
>          if (c->role & (BDRV_CHILD_DATA | BDRV_CHILD_FILTERED | BDRV_CHILD_COW))
>          {
> -            bdrv_refresh_limits(c->bs, tran, errp);
> -            if (*errp) {
> -                return;
> -            }
>              bdrv_merge_limits(&bs->bl, &c->bs->bl);
>              have_limits = true;
>          }
> -- 
> 2.34.1
> 
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org