linux-kernel - Re: processes hung after sys_renameat, and 'missing' processes

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Message-ID: <20120603235904.GS30000@ZenIV.linux.org.uk>
Date:	Mon, 4 Jun 2012 00:59:04 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	"J. Bruce Fields" <bfields@...hat.com>
Subject: Re: processes hung after sys_renameat, and 'missing' processes

On Mon, Jun 04, 2012 at 12:40:42AM +0100, Al Viro wrote:
> On Mon, Jun 04, 2012 at 12:28:20AM +0100, Al Viro wrote:
> 
> > Everything in lock_rename() appears to be at lock_rename+0x3e.  Unless
> > there's a really huge amount of filesystems on that box, this has to
> > be
> >                 mutex_lock_nested(&p1->d_inode->i_mutex, I_MUTEX_PARENT);
> > and everything on that sucker is not holding any locks yet.  IOW, that's
> > the tail hanging off whatever deadlock is there.
> 
> Er...  After another look, probably not - it's ->s_vfs_rename_mutex,
> so we are seeing one cross-directory rename stuck on something with
> all subsequent ones blocked on attempt to grab said mutex.
> 
> The interesting one is the guy stuck at lock_rename+0xc9/0xf0, everything
> else in lock_rename() is the consequence.

BTW, another suspicious patch is d_splice_alias() one; note that if we
_ever_ pick a dentry that isn't disconnected, we are deeply fucked.
d_move() without the old parent locked is a Bad Thing(tm).  I don't see
how that could've triggered without another bug somewhere, but what's
happening in d_splice_alias() right now is wrong.
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