linux-kernel - Re: [RFC v2 0/5] Non-blockling buffered fs read (page cache only)

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Date:	Mon, 22 Sep 2014 07:25:19 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Jonathan Corbet <corbet@....net>
Cc:	Milosz Tanski <milosz@...in.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@...radead.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	linux-aio@...ck.org, Mel Gorman <mgorman@...e.de>,
	Volker Lendecke <Volker.Lendecke@...net.de>,
	Tejun Heo <tj@...nel.org>, Jeff Moyer <jmoyer@...hat.com>,
	Theodore Ts'o <tytso@....edu>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC v2 0/5] Non-blockling buffered fs read (page cache only)

On Mon, Sep 22, 2014 at 10:12:21AM -0400, Jonathan Corbet wrote:
> So I'm not contesting this, but I am genuinely curious: do you think
> there are applications out there requesting non-blocking behavior on
> regular files that will then break if they actually get non-blocking
> behavior?  I don't suppose you have an example?

You only have to look as far as Samba, but Jeff quoted an old lkml
post earlier that had other examples.

source3/smbd/open.c:


	if (first_open_attempt && lp_kernel_oplocks(SNUM(conn))) {
		/*
		 * With kernel oplocks the open breaking an oplock
		 * blocks until the oplock holder has given up the
		 * oplock or closed the file. We prevent this by first
		 * trying to open the file with O_NONBLOCK (see "man
		 * fcntl" on Linux). For the second try, triggered by
		 * an oplock break response, we do not need this
		 * anymore.
		 *
		 * This is true under the assumption that only Samba
		 * requests kernel oplocks. Once someone else like
		 * NFSv4 starts to use that API, we will have to
		 * modify this by communicating with the NFSv4 server.
		 */
		flags2 |= O_NONBLOCK;
	}
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