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Re: [Qemu-devel] [PATCH 06/17] block/nbd-client: fix nbd_read_reply_entr

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-devel] [PATCH 06/17] block/nbd-client: fix nbd_read_reply_entry
Date: Mon, 7 Aug 2017 15:56:24 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1

07.08.2017 14:52, Eric Blake wrote:
On 08/04/2017 10:14 AM, Vladimir Sementsov-Ogievskiy wrote:
Set reply.handle to 0 on error path to prevent normal path of

Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
  block/nbd-client.c | 1 +
  1 file changed, 1 insertion(+)
Can you document a case where not fixing this would be an observable bug
(even if it requires using gdb and single-stepping between client and
server to make what is otherwise a racy situation easy to see)?  I'm
trying to figure out if this is 2.10 material.

it is simple enough:

run qemu-nbd in gdb, set break on nbd_send_reply, and when it shoot s,
next up to "stl_be_p(buf, NBD_REPLY_MAGIC);" and after it do "call stl_be_p(buf, 1000)"

run qemu-io with some read in gdb, set break on
br block/nbd-client.c:83

( it is break; after failed nbd_receive_reply call)

and on
br block/nbd-client.c:170

(it is in nbd_co_receive_reply after yield)

on first break we will be sure that  nbd_receive_reply failed,
on second we will be sure by
(gdb) p s->reply
$1 = {handle = 93825000680144, error = 0}
(gdb) p request->handle
$2 = 93825000680144

that we are on normal receiving path.

diff --git a/block/nbd-client.c b/block/nbd-client.c
index dc19894a7c..0c88d84de6 100644
--- a/block/nbd-client.c
+++ b/block/nbd-client.c
@@ -107,6 +107,7 @@ static coroutine_fn void nbd_read_reply_entry(void *opaque)
+ s->reply.handle = 0;
      s->read_reply_co = NULL;

Best regards,

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