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From: "J. Bruce Fields" <bfields@fieldses.org>
To: Christoph Hellwig <hch@lst.de>
Cc: xfs@oss.sgi.com, viro@zeniv.linux.org.uk, linux-nfs@vger.kernel.org
Subject: Re: [PATCH] xfs: unlock i_mutex in xfs_break_layouts
Date: Wed, 8 Apr 2015 14:16:51 -0400	[thread overview]
Message-ID: <20150408181651.GA3916@fieldses.org> (raw)
In-Reply-To: <20150408162104.GC16052@lst.de>

On Wed, Apr 08, 2015 at 06:21:04PM +0200, Christoph Hellwig wrote:
> On Tue, Apr 07, 2015 at 05:07:47PM -0400, J. Bruce Fields wrote:
> > On Tue, Apr 07, 2015 at 05:35:44PM +0200, Christoph Hellwig wrote:
> > > We want to drop all I/O path locks when recalling layouts, and that includes
> > > i_mutex for the write path.  Without this we get stuck processe when recalls
> > > take too long.
> > 
> > Also if the writer is an nfsd thread than we'd rather just error out
> > than wait.

(To be clear: ACK to this patch as far as I'm concerned, I've got
another concern but we need this fix regardless.)

> We have no way to know we are called by nfsd here unfortunately.

I was imagining the possible deadlock here as mostly theoretical, but
now that I think of it it doesn't sound unlikely at all:

	- file is under heavy write load
	- conflicting operation breaks layout
	- nfsd threads all block in writes to that file
	- no nfsd threads available to service layout return
	- recall times out, client fenced.

Ugh.

--b.

WARNING: multiple messages have this Message-ID (diff)
From: "J. Bruce Fields" <bfields@fieldses.org>
To: Christoph Hellwig <hch@lst.de>
Cc: linux-nfs@vger.kernel.org, viro@zeniv.linux.org.uk, xfs@oss.sgi.com
Subject: Re: [PATCH] xfs: unlock i_mutex in xfs_break_layouts
Date: Wed, 8 Apr 2015 14:16:51 -0400	[thread overview]
Message-ID: <20150408181651.GA3916@fieldses.org> (raw)
In-Reply-To: <20150408162104.GC16052@lst.de>

On Wed, Apr 08, 2015 at 06:21:04PM +0200, Christoph Hellwig wrote:
> On Tue, Apr 07, 2015 at 05:07:47PM -0400, J. Bruce Fields wrote:
> > On Tue, Apr 07, 2015 at 05:35:44PM +0200, Christoph Hellwig wrote:
> > > We want to drop all I/O path locks when recalling layouts, and that includes
> > > i_mutex for the write path.  Without this we get stuck processe when recalls
> > > take too long.
> > 
> > Also if the writer is an nfsd thread than we'd rather just error out
> > than wait.

(To be clear: ACK to this patch as far as I'm concerned, I've got
another concern but we need this fix regardless.)

> We have no way to know we are called by nfsd here unfortunately.

I was imagining the possible deadlock here as mostly theoretical, but
now that I think of it it doesn't sound unlikely at all:

	- file is under heavy write load
	- conflicting operation breaks layout
	- nfsd threads all block in writes to that file
	- no nfsd threads available to service layout return
	- recall times out, client fenced.

Ugh.

--b.

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  reply	other threads:[~2015-04-08 18:16 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-04-07 15:35 [PATCH] xfs: unlock i_mutex in xfs_break_layouts Christoph Hellwig
2015-04-07 15:35 ` Christoph Hellwig
2015-04-07 21:07 ` J. Bruce Fields
2015-04-07 21:07   ` J. Bruce Fields
2015-04-08 16:21   ` Christoph Hellwig
2015-04-08 16:21     ` Christoph Hellwig
2015-04-08 18:16     ` J. Bruce Fields [this message]
2015-04-08 18:16       ` J. Bruce Fields
2015-04-07 22:19 ` Dave Chinner
2015-04-07 22:19   ` Dave Chinner
2015-04-08 16:24   ` Christoph Hellwig
2015-04-08 16:24     ` Christoph Hellwig

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