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Re: [PATCH] xfs: xfs_alloc_fix_minleft can underflow near ENOSPC

To: Mark Tinguely <tinguely@xxxxxxx>
Subject: Re: [PATCH] xfs: xfs_alloc_fix_minleft can underflow near ENOSPC
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Tue, 17 Feb 2015 10:17:16 +1100
Cc: "Michael L. Semon" <mlsemon35@xxxxxxxxx>, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <54E22A76.40106@xxxxxxx>
References: <1423782857-11800-1-git-send-email-david@xxxxxxxxxxxxx> <54DE8B6D.8010401@xxxxxxx> <20150214232951.GW4251@dastard> <54E16667.1050200@xxxxxxxxx> <54E22A76.40106@xxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Mon, Feb 16, 2015 at 11:35:50AM -0600, Mark Tinguely wrote:
> Thanks Michael, you don't need to hold your test box for me. I do
> have a way to recreate these ABBA AGF buffer allocation deadlocks
> and understand the whys and hows very well. I don't have a community
> way to make a xfstest for it but I think your test is getting close.

If you know what is causing them, then please explain how it occurs
and how you think it needs to be fixed. Just telling us that you know
something that we don't doesn't help us solve the problem. :(

In general, the use of the args->firstblock is supposed to avoid the
ABBA locking order issues with multiple allocations in the one
transaction by preventing AG selection loops from looping back into
AGs with a lower index than the first allocation that was made.

So if you are seeing deadlocks, then it may be that we aren't
following this constraint correctly in all locations....

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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