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

[Mark Tinguely]

On 11/19/13 16:37, Dave Chinner wrote:
> From: Dave Chinner<dchinner at 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 at 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.