linux-kernel - Re: [ANNOUNCE] 3.2.9-rt16

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Date:	Tue, 6 Mar 2012 15:36:19 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>
Subject: Re: [ANNOUNCE] 3.2.9-rt16

On Tue, 6 Mar 2012, Steven Rostedt wrote:

> Note, yesterday while running some stress tests I hit a live lock here:
> ... 
> When it fails to grab either the inode->i_lock or the parent->d_lock it
> returns back to dput() and dput() will retry. We get into another one of
> these cases where we can spin blocking the holder of the locks.

Nasty.
 
> I experimented with adding a grab lock of the inode->i_lock or
> parent->d_lock if they existed (required initializing parent to NULL),
> which seemed to help a lot, but then eventually it locked up. As I'm not
> sure its safe to grab them straight here even after we release the
> dentry->d_lock. I'll have to enable full lockdep to see if this breaks
> the ordering.
> 
> I haven't looked too deeply into this code yet, but I'm assuming that
> dput() can be called where we can't just take the inode or parent lock?

If you read the top of fs/dcache.c then you find an explanation of the
lock ordering. This code takes the locks in reverse order. That's why
it uses trylock.

Thanks,

	tglx
    
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