Re: mq: INFO: possible circular locking dependency detected

[Jan Kara <jack@suse.cz>]

On Wed 08-08-12 15:54:39, Wu Fengguang wrote:
> On Wed, Aug 08, 2012 at 03:17:38PM +0800, Fengguang Wu wrote:
> > On Tue, Aug 07, 2012 at 07:39:55AM +0100, Al Viro wrote:
> > > On Tue, Aug 07, 2012 at 01:04:12PM +0800, Fengguang Wu wrote:
> > > 
> > > > FYI, here is a different back trace on that commit.
> > > > 
> > > > [    3.255043] ======================================================
> > > > [    3.255052] [ INFO: possible circular locking dependency detected ]
> > > > [    3.255052] 3.5.0-rc6-bisect-00355-geb04c28 #4 Not tainted         
> > > > [    3.255052] -------------------------------------------------------
> > > > [    3.255052] init/1 is trying to acquire lock:
> > > > [    3.255052]  (&mm->mmap_sem){++++++}, at: [<ffffffff81180d00>] might_fault+0x70/0xe0
> > > > [    3.255052]
> > > > [    3.255052] but task is already holding lock:
> > > > [    3.255052]  (&sb->s_type->i_mutex_key){+.+.+.}, at: [<ffffffff811d191e>] vfs_readdir+0x6e/0x130
> > > 
> > > Do you see any similar with the _next_ commit?
> > 
> > Stress tests show that the next commit is free from both the "circular
> > locking dependency" issues.
> 
> Sorry.. but this still remains for commit 5d37e9e6("fs: Skip atime
> update on frozen filesystem"):
>
> [  175.588560] [ INFO: possible circular locking dependency detected ]
> [  175.588560] 3.5.0-rc6-bisect-00356-g5d37e9e6 #46 Not tainted
> [  175.588560] -------------------------------------------------------
> [  175.588560] trinity-child0/493 is trying to acquire lock:
> [  175.588560]  (&sb->s_type->i_mutex_key#14){+.+.+.}, at: [<ffffffff811cc4fe>] vfs_unlink+0x6e/0x1d0
> [  175.588560] 
> [  175.588560] but task is already holding lock:
> [  175.588560]  (sb_writers#11){.+.+.+}, at: [<ffffffff811e40af>] mnt_want_write+0x2f/0x90
> [  175.588560] 
> [  175.588560] which lock already depends on the new lock.
  OK, this is a theoretical deadlock since mqueue (virtual) filesystem cannot
really be frozen. But we should convert the mqueue code to deal with new
lock ordering of i_mutex and mnt_want_write() anyway to make lockdep happy
if anything... That file somehow escaped my conversion efforts. I'll send a
fix tomorrow.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR