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
requirements:
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 */
qemu_coroutine_yield();
- } else {
+ } else if (done) {
+ /* XXX this is needed by nbd_reply_ready. */
qio_channel_wait(ioc,
do_read ? G_IO_IN : G_IO_OUT);
+ } else {
+ return -EAGAIN;
}