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

From: Eric Sandeen <sandeen@sandeen.net>
To: Mark Tinguely <tinguely@sgi.com>, 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 17:24:23 -0600	[thread overview]
Message-ID: <528BF327.2050802@sandeen.net> (raw)
In-Reply-To: <528BEF71.1000607@sgi.com>

On 11/19/13, 5:08 PM, Mark Tinguely wrote:
> 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, can you remind us of your testcase that produces this?
(sorry, I guess I should search for that old thread...)

Thanks,
-Eric

> --Mark.

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