[Qemu-devel] [PATCH] block: Fix invalidate_cache error path for parent activation

[Qemu-devel] [PATCH] block: Fix invalidate_cache error path for parent activation

[Kevin Wolf]

bdrv_co_invalidate_cache() clears the BDRV_O_INACTIVE flag before
actually activating a node so that the correct permissions etc. are
taken. In case of errors, the flag must be restored so that the next
call to bdrv_co_invalidate_cache() retries activation.

Restoring the flag was missing in the error path for a failed
parent->role->activate() call. The consequence is that this attempt to
activate all images correctly fails because we still set errp, however
on the next attempt BDRV_O_INACTIVE is already clear, so we return
success without actually retrying the failed action.

An example where this is observable in practice is migration to a QEMU
instance that has a raw format block node attached to a guest device
with share-rw=off (the default) while another process holds
BLK_PERM_WRITE for the same image. In this case, all activation steps
before parent->role->activate() succeed because raw can tolerate other
writers to the image. Only the parent callback (in particular
blk_root_activate()) tries to implement the share-rw=on property and
requests exclusive write permissions. This fails when the migration
completes and correctly displays an error. However, a manual 'cont' will
incorrectly resume the VM without calling blk_root_activate() again.

This case is described in more detail in the following bug report:
https://bugzilla.redhat.com/show_bug.cgi?id=1531888

Fix this by correctly restoring the BDRV_O_INACTIVE flag in the error
path.

Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/block.c b/block.c
index 0eba8ebe5c..7f5c9bb02b 100644
--- a/block.c
+++ b/block.c
@@ -4549,6 +4549,7 @@ static void coroutine_fn bdrv_co_invalidate_cache(BlockDriverState *bs,
         if (parent->role->activate) {
             parent->role->activate(parent, &local_err);
             if (local_err) {
+                bs->open_flags |= BDRV_O_INACTIVE;
                 error_propagate(errp, local_err);
                 return;
             }
-- 
2.20.1

Re: [Qemu-devel] [Qemu-block] [PATCH] block: Fix invalidate_cache error path for parent activation

[Stefan Hajnoczi]

[-- Attachment #1: Type: text/plain, Size: 1801 bytes --]

On Thu, Jan 31, 2019 at 03:31:51PM +0100, Kevin Wolf wrote:
> bdrv_co_invalidate_cache() clears the BDRV_O_INACTIVE flag before
> actually activating a node so that the correct permissions etc. are
> taken. In case of errors, the flag must be restored so that the next
> call to bdrv_co_invalidate_cache() retries activation.
> 
> Restoring the flag was missing in the error path for a failed
> parent->role->activate() call. The consequence is that this attempt to
> activate all images correctly fails because we still set errp, however
> on the next attempt BDRV_O_INACTIVE is already clear, so we return
> success without actually retrying the failed action.
> 
> An example where this is observable in practice is migration to a QEMU
> instance that has a raw format block node attached to a guest device
> with share-rw=off (the default) while another process holds
> BLK_PERM_WRITE for the same image. In this case, all activation steps
> before parent->role->activate() succeed because raw can tolerate other
> writers to the image. Only the parent callback (in particular
> blk_root_activate()) tries to implement the share-rw=on property and
> requests exclusive write permissions. This fails when the migration
> completes and correctly displays an error. However, a manual 'cont' will
> incorrectly resume the VM without calling blk_root_activate() again.
> 
> This case is described in more detail in the following bug report:
> https://bugzilla.redhat.com/show_bug.cgi?id=1531888
> 
> Fix this by correctly restoring the BDRV_O_INACTIVE flag in the error
> path.
> 
> Cc: qemu-stable@nongnu.org
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  block.c | 1 +
>  1 file changed, 1 insertion(+)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 455 bytes --]

Re: [Qemu-devel] [PATCH] block: Fix invalidate_cache error path for parent activation

[Markus Armbruster]

Kevin Wolf <kwolf@redhat.com> writes:

> bdrv_co_invalidate_cache() clears the BDRV_O_INACTIVE flag before
> actually activating a node so that the correct permissions etc. are
> taken. In case of errors, the flag must be restored so that the next
> call to bdrv_co_invalidate_cache() retries activation.
>
> Restoring the flag was missing in the error path for a failed
> parent->role->activate() call. The consequence is that this attempt to
> activate all images correctly fails because we still set errp, however
> on the next attempt BDRV_O_INACTIVE is already clear, so we return
> success without actually retrying the failed action.
>
> An example where this is observable in practice is migration to a QEMU
> instance that has a raw format block node attached to a guest device
> with share-rw=off (the default) while another process holds
> BLK_PERM_WRITE for the same image. In this case, all activation steps
> before parent->role->activate() succeed because raw can tolerate other
> writers to the image. Only the parent callback (in particular
> blk_root_activate()) tries to implement the share-rw=on property and
> requests exclusive write permissions. This fails when the migration
> completes and correctly displays an error. However, a manual 'cont' will
> incorrectly resume the VM without calling blk_root_activate() again.
>
> This case is described in more detail in the following bug report:
> https://bugzilla.redhat.com/show_bug.cgi?id=1531888
>
> Fix this by correctly restoring the BDRV_O_INACTIVE flag in the error
> path.
>
> Cc: qemu-stable@nongnu.org
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  block.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/block.c b/block.c
> index 0eba8ebe5c..7f5c9bb02b 100644
> --- a/block.c
> +++ b/block.c
> @@ -4549,6 +4549,7 @@ static void coroutine_fn bdrv_co_invalidate_cache(BlockDriverState *bs,
>          if (parent->role->activate) {
>              parent->role->activate(parent, &local_err);
>              if (local_err) {
> +                bs->open_flags |= BDRV_O_INACTIVE;
>                  error_propagate(errp, local_err);
>                  return;
>              }

Tested-by: Markus Armbruster <armbru@redhat.com>