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Date: Wed, 28 Aug 2024 06:51:13 +0200
From: Christoph Hellwig <hch@lst.de>
To: "Darrick J. Wong" <djwong@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>,
	Chandan Babu R <chandan.babu@oracle.com>,
	Christian Brauner <brauner@kernel.org>, linux-xfs@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 04/10] iomap: zeroing already holds invalidate_lock in
 iomap_file_buffered_write_punch_delalloc
Message-ID: <20240828045113.GC31463@lst.de>
References: <20240827051028.1751933-1-hch@lst.de> <20240827051028.1751933-5-hch@lst.de> <20240827162804.GY865349@frogsfrogsfrogs>
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User-Agent: Mutt/1.5.17 (2007-11-01)

On Tue, Aug 27, 2024 at 09:28:04AM -0700, Darrick J. Wong wrote:
> On Tue, Aug 27, 2024 at 07:09:51AM +0200, Christoph Hellwig wrote:
> > All callers of iomap_zero_range already hold invalidate_lock, so we can't
> > take it again in iomap_file_buffered_write_punch_delalloc.
> 
> What about the xfs_zero_range call in xfs_file_write_checks?  AFAICT we
> don't hold the invalidate lock there.  Did I misread that?

No, I think you're right.  My testing just never managed to hit a short
zero while doing the write prep.

I guess I'll need to do something more complicated than the zero flag
then. I initially added a new flag just for that and then (wrongly as you
pointed out) that I don't need it after all.

> Also, would nested takings of the invalidate lock cause a livelock?  Or
> is this actually quite broken now?

It is a cold, hard deadlock.