From mboxrd@z Thu Jan  1 00:00:00 1970
From: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: [PATCH] [RFC] Fix deadlock on regular nonseekable files
Date: Fri, 20 Mar 2015 14:46:34 +0000
Message-ID: <20150320144634.GY29656@ZenIV.linux.org.uk>
References: <CANZZXhCCUh12_1gt0ppc4cMMR-BUsq4rySqNihsEJziOGYbh3A@mail.gmail.com>
 <20150320134232.GX29656@ZenIV.linux.org.uk>
 <CANZZXhBNSVmFTsteG7AAHuuYK7qU8J2LaHfcO16-2mcc+7kxag@mail.gmail.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	David Vrabel <david.vrabel@citrix.com>,
	Marek =?iso-8859-1?Q?Marczykowski-G=F3recki?=
	<marmarek@invisiblethingslab.com>,
	Iurii Konovalenko <iurii.konovalenko@globallogic.com>,
	Ian Campbell <ian.campbell@citrix.com>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Andrii Anisov <andrii.anisov@globallogic.com>,
	Artem Mygaiev <artem.mygaiev@globallogic.com>
To: Vitaly Chernooky <vitalii.chernookyi@globallogic.com>
Return-path: <linux-fsdevel-owner@vger.kernel.org>
Received: from zeniv.linux.org.uk ([195.92.253.2]:59256 "EHLO
	ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1750967AbbCTOqi (ORCPT
	<rfc822;linux-fsdevel@vger.kernel.org>);
	Fri, 20 Mar 2015 10:46:38 -0400
Content-Disposition: inline
In-Reply-To: <CANZZXhBNSVmFTsteG7AAHuuYK7qU8J2LaHfcO16-2mcc+7kxag@mail.gmail.com>
Sender: linux-fsdevel-owner@vger.kernel.org
List-ID: <linux-fsdevel.vger.kernel.org>

On Fri, Mar 20, 2015 at 04:22:11PM +0200, Vitaly Chernooky wrote:
> > AFAICS, any threads blocked on f_pos_lock are not holding anything else and
> > cannot impede the rest.  What am I missing here?
> 
> As far as I understand it is true only for files on regular filesystem
> like ext4. Let's to see how XEN guys run into trouble with that
> f_pos_lock:

What does it have to do with filesystem type involved?  The only place that
takes f_pos_lock is __fdget_pos(), with only one caller in the entire tree -
fdget_pos().  Which is static in fs/read_write.c and all its callers are
in right in the beginning of sys_<something>.

Is it just that they have read() on procfs file blocked waiting for something
and a bunch of other read/write on the same descriptor blocked on ->f_pos_mutex
waiting for that sucker to finish?

Then basically they are asking to waive XSI 2.9.7 for that file - behaviour
*is* required by POSIX.

What file it is and what is the first read() (or write(), whatever) blocked on?
Stack traces would be useful for the latter...