[PATCH] nbd: Fix regression with multiple meta contexts

[PATCH] nbd: Fix regression with multiple meta contexts

[Eric Blake]

Detected by a hang in the libnbd testsuite.  If a client requests
multiple meta contexts (both base:allocation and qemu:dirty-bitmap:x)
at the same time, our attempt to silence a false-positive warning
about a potential uninitialized variable introduced botched logic: we
were short-circuiting the second context, and never sending the
NBD_REPLY_FLAG_DONE.  Combining two 'if' into one 'if/else' in
bdf200a55 was wrong (I'm a bit embarrassed that such a change was my
initial suggestion after the v1 patch, then I did not review the v2
patch that actually got committed). Revert that, and instead silence
the false positive warning by replacing 'return ret' with 'return 0'
(the value it always has at that point in the code, even though it
eluded the deduction abilities of the robot that reported the false
positive).

Fixes: bdf200a5535
Signed-off-by: Eric Blake <eblake@redhat.com>
---

It's never fun when a regression is caused by a patch taken through
qemu-trivial, proving that the patch was not trivial after all.

 nbd/server.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/nbd/server.c b/nbd/server.c
index 87fcd2e7bfac..11a31094ff83 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -2384,15 +2384,23 @@ static coroutine_fn int nbd_handle_request(NBDClient *client,
                                                !client->export_meta.bitmap,
                                                NBD_META_ID_BASE_ALLOCATION,
                                                errp);
-            } else {              /* client->export_meta.bitmap */
+                if (ret < 0) {
+                    return ret;
+                }
+            }
+
+            if (client->export_meta.bitmap) {
                 ret = nbd_co_send_bitmap(client, request->handle,
                                          client->exp->export_bitmap,
                                          request->from, request->len,
                                          dont_fragment,
                                          true, NBD_META_ID_DIRTY_BITMAP, errp);
+                if (ret < 0) {
+                    return ret;
+                }
             }

-            return ret;
+            return 0;
         } else {
             return nbd_send_generic_reply(client, request->handle, -EINVAL,
                                           "CMD_BLOCK_STATUS not negotiated",
-- 
2.24.1

Re: [PATCH] nbd: Fix regression with multiple meta contexts

[Laurent Vivier]

Le 06/02/2020 à 18:38, Eric Blake a écrit :
> Detected by a hang in the libnbd testsuite.  If a client requests
> multiple meta contexts (both base:allocation and qemu:dirty-bitmap:x)
> at the same time, our attempt to silence a false-positive warning
> about a potential uninitialized variable introduced botched logic: we
> were short-circuiting the second context, and never sending the
> NBD_REPLY_FLAG_DONE.  Combining two 'if' into one 'if/else' in
> bdf200a55 was wrong (I'm a bit embarrassed that such a change was my
> initial suggestion after the v1 patch, then I did not review the v2
> patch that actually got committed). Revert that, and instead silence
> the false positive warning by replacing 'return ret' with 'return 0'
> (the value it always has at that point in the code, even though it
> eluded the deduction abilities of the robot that reported the false
> positive).
> 
> Fixes: bdf200a5535
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
> 
> It's never fun when a regression is caused by a patch taken through
> qemu-trivial, proving that the patch was not trivial after all.

trivial doesn't mean not reviewed...
The patch v1 was trivial, the v2 wasn't.

> 
>  nbd/server.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/nbd/server.c b/nbd/server.c
> index 87fcd2e7bfac..11a31094ff83 100644
> --- a/nbd/server.c
> +++ b/nbd/server.c
> @@ -2384,15 +2384,23 @@ static coroutine_fn int nbd_handle_request(NBDClient *client,
>                                                 !client->export_meta.bitmap,
>                                                 NBD_META_ID_BASE_ALLOCATION,
>                                                 errp);
> -            } else {              /* client->export_meta.bitmap */
> +                if (ret < 0) {
> +                    return ret;
> +                }
> +            }
> +
> +            if (client->export_meta.bitmap) {
>                  ret = nbd_co_send_bitmap(client, request->handle,
>                                           client->exp->export_bitmap,
>                                           request->from, request->len,
>                                           dont_fragment,
>                                           true, NBD_META_ID_DIRTY_BITMAP, errp);
> +                if (ret < 0) {
> +                    return ret;
> +                }
>              }
> 
> -            return ret;
> +            return 0;
>          } else {
>              return nbd_send_generic_reply(client, request->handle, -EINVAL,
>                                            "CMD_BLOCK_STATUS not negotiated",
> 

Reviewed-by: Laurent Vivier <laurent@vivier.eu>

Re: [PATCH] nbd: Fix regression with multiple meta contexts

[Laurent Vivier]

Le 06/02/2020 à 18:38, Eric Blake a écrit :
> Detected by a hang in the libnbd testsuite.  If a client requests
> multiple meta contexts (both base:allocation and qemu:dirty-bitmap:x)
> at the same time, our attempt to silence a false-positive warning
> about a potential uninitialized variable introduced botched logic: we
> were short-circuiting the second context, and never sending the
> NBD_REPLY_FLAG_DONE.  Combining two 'if' into one 'if/else' in
> bdf200a55 was wrong (I'm a bit embarrassed that such a change was my
> initial suggestion after the v1 patch, then I did not review the v2
> patch that actually got committed). Revert that, and instead silence
> the false positive warning by replacing 'return ret' with 'return 0'
> (the value it always has at that point in the code, even though it
> eluded the deduction abilities of the robot that reported the false
> positive).
> 
> Fixes: bdf200a5535
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
> 
> It's never fun when a regression is caused by a patch taken through
> qemu-trivial, proving that the patch was not trivial after all.

Do you want this one be merged using the trivial branch?

Thanks,
Laurent

Re: [PATCH] nbd: Fix regression with multiple meta contexts

[Eric Blake]

On 2/12/20 3:24 AM, Laurent Vivier wrote:
> Le 06/02/2020 à 18:38, Eric Blake a écrit :
>> Detected by a hang in the libnbd testsuite.  If a client requests
>> multiple meta contexts (both base:allocation and qemu:dirty-bitmap:x)
>> at the same time, our attempt to silence a false-positive warning
>> about a potential uninitialized variable introduced botched logic: we
>> were short-circuiting the second context, and never sending the
>> NBD_REPLY_FLAG_DONE.  Combining two 'if' into one 'if/else' in
>> bdf200a55 was wrong (I'm a bit embarrassed that such a change was my
>> initial suggestion after the v1 patch, then I did not review the v2
>> patch that actually got committed). Revert that, and instead silence
>> the false positive warning by replacing 'return ret' with 'return 0'
>> (the value it always has at that point in the code, even though it
>> eluded the deduction abilities of the robot that reported the false
>> positive).
>>
>> Fixes: bdf200a5535
>> Signed-off-by: Eric Blake <eblake@redhat.com>
>> ---
>>
>> It's never fun when a regression is caused by a patch taken through
>> qemu-trivial, proving that the patch was not trivial after all.
> 
> Do you want this one be merged using the trivial branch?

Up to you; I'm also fine taking it through my NBD tree as I have a few 
other NBD patches landing soon.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

Re: [PATCH] nbd: Fix regression with multiple meta contexts

[Laurent Vivier]

Le 12/02/2020 à 13:10, Eric Blake a écrit :
> On 2/12/20 3:24 AM, Laurent Vivier wrote:
>> Le 06/02/2020 à 18:38, Eric Blake a écrit :
>>> Detected by a hang in the libnbd testsuite.  If a client requests
>>> multiple meta contexts (both base:allocation and qemu:dirty-bitmap:x)
>>> at the same time, our attempt to silence a false-positive warning
>>> about a potential uninitialized variable introduced botched logic: we
>>> were short-circuiting the second context, and never sending the
>>> NBD_REPLY_FLAG_DONE.  Combining two 'if' into one 'if/else' in
>>> bdf200a55 was wrong (I'm a bit embarrassed that such a change was my
>>> initial suggestion after the v1 patch, then I did not review the v2
>>> patch that actually got committed). Revert that, and instead silence
>>> the false positive warning by replacing 'return ret' with 'return 0'
>>> (the value it always has at that point in the code, even though it
>>> eluded the deduction abilities of the robot that reported the false
>>> positive).
>>>
>>> Fixes: bdf200a5535
>>> Signed-off-by: Eric Blake <eblake@redhat.com>
>>> ---
>>>
>>> It's never fun when a regression is caused by a patch taken through
>>> qemu-trivial, proving that the patch was not trivial after all.
>>
>> Do you want this one be merged using the trivial branch?
> 
> Up to you; I'm also fine taking it through my NBD tree as I have a few
> other NBD patches landing soon.
> 

For the moment, I have only one patch in my queue so I think you can
take it.

Thanks,
Laurent