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Re: [PATCH] xfs: prevent spurious "head behind tail" warnings

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH] xfs: prevent spurious "head behind tail" warnings
From: Mark Tinguely <tinguely@xxxxxxx>
Date: Tue, 19 Nov 2013 17:08:33 -0600
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <1384900659-22215-1-git-send-email-david@xxxxxxxxxxxxx>
References: <1384900659-22215-1-git-send-email-david@xxxxxxxxxxxxx>
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On 11/19/13 16:37, Dave Chinner wrote:
From: Dave Chinner<dchinner@xxxxxxxxxx>

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@xxxxxxxxxx>
---

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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