Re: [Qemu-block] [PATCH v2 2/7] block/nbd-client: exit reply-reading coroutine on incorrect handle

[Eric Blake]

On 09/18/2017 08:59 AM, Vladimir Sementsov-Ogievskiy wrote:
> Check reply-handle == request-handle in the same place, where
> recv coroutine number is calculated from reply->handle and it's
> correctness checked - in nbd_read_reply_entry.
> 
> Also finish nbd_read_reply_entry in case of reply-handle !=
> request-handle in the same way as in case of incorrect reply-handle.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> ---
>  block/nbd-client.h | 1 +
>  block/nbd-client.c | 8 ++++++--
>  2 files changed, 7 insertions(+), 2 deletions(-)

On second thought:

> +++ b/block/nbd-client.c
> @@ -92,7 +92,9 @@ static coroutine_fn void nbd_read_reply_entry(void *opaque)
>          i = HANDLE_TO_INDEX(s, s->reply.handle);
>          if (i >= MAX_NBD_REQUESTS ||
>              !s->requests[i].coroutine ||
> -            !s->requests[i].receiving) {
> +            !s->requests[i].receiving ||
> +            s->reply.handle != s->requests[i].request->handle)

How can this condition ever be false?  s->reply.handle only ever
contains two values: 0 when it is being reused for the next iteration of
the loop, or the (munged) address of the request pointer.  But we are
careful that we never write s->reply.handle = 0 until after
s->requests[i].receiving is false.  So we will never see s->reply.handle
equal to 0 here.  (It may have been possible prior to the introduction
of reply.receiving, in commit 40f4a218, but I'm not seeing it now)

If I'm right, then this patch can be simplified - we don't need to track
s.requests[i].request, and can merely:

> @@ -189,9 +192,10 @@ static int nbd_co_receive_reply(NBDClientSession *s,
>      s->requests[i].receiving = true;
>      qemu_coroutine_yield();
>      s->requests[i].receiving = false;
> -    if (s->reply.handle != request->handle || !s->ioc || s->quit) {

shorten this conditional to remove the now-dead condition.

> +    if (!s->ioc || s->quit) {
>          ret = -EIO;
>      } else {
> +        assert(s->reply.handle == request->handle);
>          ret = -s->reply.error;
>          if (qiov && s->reply.error == 0) {
>              assert(request->len == iov_size(qiov->iov, qiov->niov));
> 

[Oh, and I just noticed HANDLE_TO_INDEX() and INDEX_TO_HANDLE() are
improperly parenthesized, although to no ill effect with current clients]

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

signature.asc
Description: OpenPGP digital signature