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Re: [Qemu-devel] [PATCH] nbd: do not hang nbd_wr_syncv if outside a coro

From: Changlong Xie
Subject: Re: [Qemu-devel] [PATCH] nbd: do not hang nbd_wr_syncv if outside a coroutine and no available data
Date: Fri, 8 Apr 2016 13:16:17 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

On 04/07/2016 07:44 PM, Paolo Bonzini wrote:
Until commit 1c778ef7 ("nbd: convert to using I/O channels for actual
socket I/O", 2016-02-16), nbd_wr_sync returned -EAGAIN this scenario.
nbd_reply_ready required these semantics because it has two conflicting

1) if a reply can be received on the socket, nbd_reply_ready needs
to read the header outside coroutine context to identify _which_
coroutine to enter to process the rest of the reply

2) on the other hand, nbd_reply_ready can find a false positive if
another thread (e.g. a VCPU thread running aio_poll) sneaks in and
calls nbd_reply_ready too.  In this case nbd_reply_ready does nothing
and expects nbd_wr_syncv to return -EAGAIN.

Currently, the solution to the first requirement is to wait in the very
rare case of a read() that doesn't retrieve the reply header in its
entirety; this is what nbd_wr_syncv does by calling qio_channel_wait().
However, the unconditional call to qio_channel_wait() breaks the second
requirement.  To fix this, the patch makes nbd_wr_syncv return -EAGAIN
if done is zero, similar to the code before commit 1c778ef7.

This is okay because NBD client-side negotiation is the only other case
that calls nbd_wr_syncv outside a coroutine, and it places the socket
in blocking mode.  On the other hand, it is a bit unpleasant to put
this in nbd_wr_syncv(), because the function is used by both client
and server.

The full fix would be to add a counter to NbdClientSession for how
many bytes have been filled in s->reply.  Then a reply can be filled
by multiple separate invocations of nbd_reply_ready and the
qio_channel_wait() call can be removed completely.  Something to
consider for 2.7...

Reported-by: Changlong Xie <address@hidden>
Cc: Daniel P. Berrange <address@hidden>
Signed-off-by: Paolo Bonzini <address@hidden>
  nbd/common.c | 5 ++++-
  1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/nbd/common.c b/nbd/common.c
index a44718c..8ddb2dd 100644
--- a/nbd/common.c
+++ b/nbd/common.c
@@ -50,9 +50,12 @@ ssize_t nbd_wr_syncv(QIOChannel *ioc,
                   * qio_channel_yield() that works with AIO contexts
                   * and consider using that in this branch */
-            } else {
+            } else if (done) {
+                /* XXX this is needed by nbd_reply_ready.  */
                                   do_read ? G_IO_IN : G_IO_OUT);
+            } else {
+                return -EAGAIN;

With this patch, nbd works well now.



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