From: Mark Tinguely <tinguely@sgi.com>
To: Dave Chinner <david@fromorbit.com>
Cc: xfs@oss.sgi.com
Subject: Re: [PATCH] xfs: prevent spurious "head behind tail" warnings
Date: Tue, 19 Nov 2013 17:08:33 -0600 [thread overview]
Message-ID: <528BEF71.1000607@sgi.com> (raw)
In-Reply-To: <1384900659-22215-1-git-send-email-david@fromorbit.com>
On 11/19/13 16:37, Dave Chinner wrote:
> From: Dave Chinner<dchinner@redhat.com>
>
> 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<dchinner@redhat.com>
> ---
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.
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next prev parent reply other threads:[~2013-11-19 23:08 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-11-19 22:37 [PATCH] xfs: prevent spurious "head behind tail" warnings Dave Chinner
2013-11-19 23:08 ` Mark Tinguely [this message]
2013-11-19 23:24 ` Eric Sandeen
2013-11-19 23:44 ` Mark Tinguely
2013-11-21 2:19 ` Dave Chinner
2013-11-21 3:16 ` Dave Chinner
2013-11-21 1:42 ` Dave Chinner
2013-11-21 13:01 ` Dave Chinner
2013-11-22 2:57 ` Dave Chinner
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