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Re: [PATCH v2 1/3] xfs: add scan owner field to xfs_eofblocks

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Subject: Re: [PATCH v2 1/3] xfs: add scan owner field to xfs_eofblocks
From: Brian Foster <bfoster@xxxxxxxxxx>
Date: Tue, 27 May 2014 08:18:11 -0400
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20140527104428.GC1440@xxxxxxxxxxxxx>
References: <1400845950-41435-1-git-send-email-bfoster@xxxxxxxxxx> <1400845950-41435-2-git-send-email-bfoster@xxxxxxxxxx> <20140527104428.GC1440@xxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Tue, May 27, 2014 at 03:44:28AM -0700, Christoph Hellwig wrote:
> On Fri, May 23, 2014 at 07:52:28AM -0400, Brian Foster wrote:
> > The scan owner field represents an optional inode number that is
> > responsible for the current scan. The purpose is to identify that an
> > inode is under iolock and as such, the iolock shouldn't be attempted
> > when trimming eofblocks. This is an internal only field.
> 
> xfs_free_eofblocks already does a trylock, and without that calling it
> from one iolock holding process to another would be a deadlock waiting
> to happen.
> 
> I have to say I'm still not very easy with iolock nesting, even if it's
> a trylock.
> 

Right... maybe I'm not parsing your point. The purpose here is to avoid
the trylock entirely. E.g., Indicate that we have already acquired the
lock and can proceed with xfs_free_eofblocks(), rather than fail a
trylock and skip (which appears to be a potential infinite loop scenario
here due to how the AG walking code handles EAGAIN).

Brian

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