From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Cooper Subject: Re: [PATCH] oxenstored: fix short-write issue Date: Mon, 2 Nov 2015 14:10:39 +0000 Message-ID: <56376EDF.8030802@citrix.com> References: <1445965809-5144-1-git-send-email-wei.liu2@citrix.com> <1446471883.3088.40.camel@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Received: from mail6.bemta3.messagelabs.com ([195.245.230.39]) by lists.xen.org with esmtp (Exim 4.72) (envelope-from ) id 1ZtFop-0000T0-4x for xen-devel@lists.xenproject.org; Mon, 02 Nov 2015 14:10:43 +0000 In-Reply-To: <1446471883.3088.40.camel@citrix.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Ian Campbell , Wei Liu , Xen-devel Cc: Samuel Thibault , Ian Jackson , David Scott List-Id: xen-devel@lists.xenproject.org On 02/11/15 13:44, Ian Campbell wrote: > On Tue, 2015-10-27 at 17:10 +0000, Wei Liu wrote: >> When oxenstored wrote to the ring, it wrote a chunk of contiguous data. >> Originally when it tried to write across ring boundary, it returned a >> short-write when there is still room. That led to stalling mini-os's >> xenstore thread at times. > What is a "short-write" in this context? > > Given data bytes 0..M I assumed it is only writing bytes 0..N and not > N+1..M because the ring boundary is at N. But what is it writing to the > ->prod ring pointer N or M? Prod gets incremented by N in this case. > > AIUI writing N should be allowed by the ring protocol, the client should > keep looking for more data until it has a complete request. > > Writing M would be a server error. Correct, and this is what is happening. The server (believes) that the ring is full, when it is not. It waits for the client to make more space in the ring, while the client is waiting for the server to complete its message in the ring, thus stalling. ~Andrew