From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dominique Martinet Subject: Re: [V9fs-developer] [PATCH] net: trans_rdma: remove unused function Date: Thu, 25 Jul 2013 21:05:06 +0200 Message-ID: <20130725190506.GA32375@nautica> References: <1374497956-32104-1-git-send-email-andi@etezian.org> <20130724.154646.2283898956674234778.davem@davemloft.net> <1374707387.29835.23.camel@x61.thuisdomein> <20130724.164514.393667021861625699.davem@davemloft.net> <20130725061411.GA8579@nautica> <20130725064802.GA12569@nautica> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Cc: Dominique Martinet , Latchesar Ionkov , pebolle@tiscali.nl, netdev@vger.kernel.org, linux-kernel , andi@etezian.org, rminnich@sandia.gov, V9FS Developers , David Miller To: Eric Van Hensbergen Return-path: Received: from nautica.notk.org ([91.121.71.147]:44512 "EHLO nautica.notk.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756160Ab3GYTFX (ORCPT ); Thu, 25 Jul 2013 15:05:23 -0400 Content-Disposition: inline In-Reply-To: Sender: netdev-owner@vger.kernel.org List-ID: Eric Van Hensbergen wrote on Thu, Jul 25, 2013 : > So, the cancel function should be used to flush any pending requests that > haven't actually been sent yet. Looking at the 9p RDMA code, it looks like > the thought was that this wasn't going to be possible. Regardless of > removing unsent requests, the flush will still be sent and if the server > processes it before the original request and sends a flush response back > then we need to clear the posted buffer. This is what rdma_cancelled is > supposed to be doing. So, the fix is to hook it into the structure -- but > looking at the code it seems like we probably need to do something more to > reclaim the buffer rather than just incrementing a counter. > > To be clear this has less to do with recovery and more to do with the > proper implementation of 9p flush semantics. By and large, those semantics > won't impact static file system users -- but if anyone is using the > transport to access synthetic filesystems or files then they'll definitely > want to have a properly implemented flush setup. The way to test this is > to get a blocking read on a remote named pipe or fifo and then ^C it. Ok, I knew about the concept of flush but didn't think a ^C would cause a -ESYSRESTART, so didn't think of that. That said, reading from, say, a fifo is an entierly local operation: the client does a walk, getattr, doesn't do anything 9p-wise, and clunks when it's done with it. As for the function needing a bit more work, there's a race, but on "normal" requests I think it is about right - the answer lays in a comment in rdma_request: /* When an error occurs between posting the recv and the send, * there will be a receive context posted without a pending request. * Since there is no way to "un-post" it, we remember it and skip * post_recv() for the next request. * So here, * see if we are this `next request' and need to absorb an excess rc. * If yes, then drop and free our own, and do not recv_post(). **/ Basically, receive buffers are sent in a queue, and we can't "retrieve" it back, so we just don't sent next one. There is one problem though - if the server handles the original request before getting the flush, the receive buffer will be consumed and we won't send a new one, so we'll starve the reception queue. I'm afraid I don't have any bright idea there... While we are on reception buffer issues, there is another problem with the queue of receive buffers, even without flush, in the following scenario: - post a buffer for tag 0, on a hanging request - post a buffer for tag 1 - reply for tag 1 will come on buffer from tag 0 - post another request with tag 1.. the buffer already is in the queue, and we don't know we can post the buffer associated with tag 0 back. I haven't found how to reproduce this perfectly yet, but a dd with blocksize 1MB and one with blocksize 10B in parallel brought the mountpoint down (and the whole server was completely unavailable for the duration of the dd - TCP sessions timed out, I even got IO errors on the local disk :D) Regards, -- Dominique Martinet