From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:45573) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aoD97-0003sZ-4y for qemu-devel@nongnu.org; Thu, 07 Apr 2016 12:51:06 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1aoD96-0001cY-4V for qemu-devel@nongnu.org; Thu, 07 Apr 2016 12:51:05 -0400 Received: from mx1.redhat.com ([209.132.183.28]:36161) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aoD95-0001cT-VZ for qemu-devel@nongnu.org; Thu, 07 Apr 2016 12:51:04 -0400 Received: from int-mx11.intmail.prod.int.phx2.redhat.com (int-mx11.intmail.prod.int.phx2.redhat.com [10.5.11.24]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 90AB96436D for ; Thu, 7 Apr 2016 16:51:03 +0000 (UTC) From: Paolo Bonzini Date: Thu, 7 Apr 2016 18:50:40 +0200 Message-Id: <1460047845-14488-11-git-send-email-pbonzini@redhat.com> In-Reply-To: <1460047845-14488-1-git-send-email-pbonzini@redhat.com> References: <1460047845-14488-1-git-send-email-pbonzini@redhat.com> Subject: [Qemu-devel] [PULL 10/15] nbd: do not hang nbd_wr_syncv if outside a coroutine and no available data List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org 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 Cc: Daniel P. Berrange Signed-off-by: Paolo Bonzini --- 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; } continue; } -- 2.5.5