linux-kernel - Re: frequent softlockups with 3.10rc6.

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Message-ID: <20130628083223.GF4165@ZenIV.linux.org.uk>
Date:	Fri, 28 Jun 2013 09:32:23 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
	Dave Jones <davej@...hat.com>, Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andrey Vagin <avagin@...nvz.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: frequent softlockups with 3.10rc6.

On Thu, Jun 27, 2013 at 10:22:45PM -1000, Linus Torvalds wrote:

> > It looks ok, but I still think it is solving the wrong problem.
> > FWIW, your optimisation has much wider application that just this
> > one place. I'll have a look to see how we can apply this approach
> > across all the inode lookup+validate code we currently have that
> > unconditionally takes the inode->i_lock....
> 
> Yes, I was looking at all the other cases that also seemed to be
> testing i_state for those "about to go away" cases.

FWIW, there's a subtle issue here - something like ext2_new_inode()
starts with allocating an inode and putting it into list (no I_NEW
yet), then decides what inumber will it have and calls insert_inode_locked(),
which sets I_NEW.  Then we proceed with initializing the inode (and
eventually do unlock_new_inode(), which removes I_NEW).  We depend on having
no pages in the pagecache of that sucker prior to insert_inode_locked() call;
you really don't want to start playing with writeback on this half-initialized
in-core inode.
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