From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from relay.sgi.com (relay1.corp.sgi.com [137.38.102.111]) by oss.sgi.com (Postfix) with ESMTP id 7DDE37F51 for ; Tue, 19 Nov 2013 17:08:35 -0600 (CST) Message-ID: <528BEF71.1000607@sgi.com> Date: Tue, 19 Nov 2013 17:08:33 -0600 From: Mark Tinguely MIME-Version: 1.0 Subject: Re: [PATCH] xfs: prevent spurious "head behind tail" warnings References: <1384900659-22215-1-git-send-email-david@fromorbit.com> In-Reply-To: <1384900659-22215-1-git-send-email-david@fromorbit.com> List-Id: XFS Filesystem from SGI List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Errors-To: xfs-bounces@oss.sgi.com Sender: xfs-bounces@oss.sgi.com To: Dave Chinner Cc: xfs@oss.sgi.com On 11/19/13 16:37, Dave Chinner wrote: > From: Dave Chinner > > When xlog_space_left() cracks the grant head and the log tail, it > does so without locking to synchronise the sampling of the > variables. It samples the grant head first, so if there is a delay > before it smaples the log tail, there is a window where the log tail > could have moved onwards and be moved past the sampled value of the > grant head. This then leads to the "xlog_space_left: head behind > tail" warning message. > > To avoid spurious output in this situation, swap the order in which > the variables are cracked. This means that the head may grant head > may move if there is a delay, but the log tail will be stable, hence > ensure the tail does not jump the head accidentally. > > While this avoids the spurious head behind tail problem, it > introduces the opposite problem - the head can move more than a full > cycle past the tail. The code already handles this case by > indicating that the log is full (i.e. zero space available) but > that's still (generally) a spurious situation. > > Hence, if we detect that the head is more than a cycle ahead of the > tail or the head is behind the tail, start the calculation again by > resampling the variables and trying again. If we get too many > resamples, then throw a warning and return a full or empty log > appropriately. > > Signed-off-by: Dave Chinner > --- I am still getting the debug message: xlog_verify_grant_tail: space > BBTOB(tail_blocks) This is a real over grant. It has been a while since I did all the tests, but basically the only way to stop it is to have a lock between checking for xlog_space_left() and actually reserving the space. I am not a fan of another band-aid on a problem that is caused because we are granting space without locks. --Mark. _______________________________________________ xfs mailing list xfs@oss.sgi.com http://oss.sgi.com/mailman/listinfo/xfs