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Re: xfs i_lock vs mmap_sem lockdep trace.

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: xfs i_lock vs mmap_sem lockdep trace.
From: Dave Jones <davej@xxxxxxxxxx>
Date: Sun, 30 Mar 2014 20:20:30 -0400
Cc: Linux Kernel <linux-kernel@xxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20140330234335.GB16336@dastard>
Mail-followup-to: Dave Jones <davej@xxxxxxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>, Linux Kernel <linux-kernel@xxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
References: <20140329223109.GA24098@xxxxxxxxxx> <20140330234335.GB16336@dastard>
User-agent: Mutt/1.5.21 (2010-09-15)
On Mon, Mar 31, 2014 at 10:43:35AM +1100, Dave Chinner wrote:
 > On Sat, Mar 29, 2014 at 06:31:09PM -0400, Dave Jones wrote:
 > > Not sure if I've reported this already (it looks familiar, though I've not 
 > > managed
 > > to find it in my sent mail folder).  This is rc8 + a diff to fix the stack 
 > > usage reports
 > > I was seeing (diff at http://paste.fedoraproject.org/89854/13210913/raw)
 > > 
 > >  ======================================================
 > >  [ INFO: possible circular locking dependency detected ]
 > >  3.14.0-rc8+ #153 Not tainted
 > >  -------------------------------------------------------
 > >  git/32710 is trying to acquire lock:
 > >   (&(&ip->i_lock)->mr_lock){++++.+}, at: [<ffffffffc03bd782>] 
 > > xfs_ilock+0x122/0x250 [xfs]
 > >  
 > > but task is already holding lock:
 > >   (&mm->mmap_sem){++++++}, at: [<ffffffffae7b816a>] 
 > > __do_page_fault+0x14a/0x610
 > > 
 > > which lock already depends on the new lock.
 > 
 > filldir on a directory inode vs page fault on regular file. Known
 > issue, definitely a false positive.

ah yeah, thought it looked familiar. I think I reported this last summer.

 > We have to change locking
 > algorithms to avoid such deficiencies of lockdep (a case of "lockdep
 > considered harmful", perhaps?) so it's not something I'm about to
 > rush...

Bummer, as it makes lockdep useless on my test box using xfs because it
disables itself after hitting this very quickly.
(I re-enabled it a couple days ago wondering why I'd left it turned off,
 chances are it was because of this)

        Dave

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