linux-kernel - Re: [PATCH] [RFC] Fix deadlock on regular nonseekable files

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Message-ID: <20150320144634.GY29656@ZenIV.linux.org.uk>
Date:	Fri, 20 Mar 2015 14:46:34 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Vitaly Chernooky <vitalii.chernookyi@...ballogic.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Vrabel <david.vrabel@...rix.com>,
	Marek Marczykowski-Górecki 
	<marmarek@...isiblethingslab.com>,
	Iurii Konovalenko <iurii.konovalenko@...ballogic.com>,
	Ian Campbell <ian.campbell@...rix.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Andrii Anisov <andrii.anisov@...ballogic.com>,
	Artem Mygaiev <artem.mygaiev@...ballogic.com>
Subject: Re: [PATCH] [RFC] Fix deadlock on regular nonseekable files

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...
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