On 2012-03-08 13:02:56, Andrew Morton wrote:
> On Thu,  8 Mar 2012 14:45:16 +0530
> "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> wrote:
> 
> > From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
> > 
> > This fix the below lockdep warning
> 
> OK, what's going on here.
> 
> >  ======================================================
> >  [ INFO: possible circular locking dependency detected ]
> >  3.3.0-rc4+ #190 Not tainted
> >  -------------------------------------------------------
> >  shared/1568 is trying to acquire lock:
> >   (&sb->s_type->i_mutex_key#12){+.+.+.}, at: [<ffffffff811efa0f>] hugetlbfs_file_mmap+0x7d/0x108
> > 
> >  but task is already holding lock:
> >   (&mm->mmap_sem){++++++}, at: [<ffffffff810f5589>] sys_mmap_pgoff+0xd4/0x12f
> > 
> >  which lock already depends on the new lock.
> > 
> > 
> >  the existing dependency chain (in reverse order) is:
> > 
> >  -> #1 (&mm->mmap_sem){++++++}:
> >         [<ffffffff8109fb8f>] lock_acquire+0xd5/0xfa
> >         [<ffffffff810ee439>] might_fault+0x6d/0x90
> >         [<ffffffff8111bc12>] filldir+0x6a/0xc2
> >         [<ffffffff81129942>] dcache_readdir+0x5c/0x222
> >         [<ffffffff8111be58>] vfs_readdir+0x76/0xac
> >         [<ffffffff8111bf6a>] sys_getdents+0x79/0xc9
> >         [<ffffffff816940a2>] system_call_fastpath+0x16/0x1b
> > 
> >  -> #0 (&sb->s_type->i_mutex_key#12){+.+.+.}:
> >         [<ffffffff8109f40a>] __lock_acquire+0xa6c/0xd60
> >         [<ffffffff8109fb8f>] lock_acquire+0xd5/0xfa
> >         [<ffffffff816916be>] __mutex_lock_common+0x48/0x350
> >         [<ffffffff81691a85>] mutex_lock_nested+0x2a/0x31
> >         [<ffffffff811efa0f>] hugetlbfs_file_mmap+0x7d/0x108
> >         [<ffffffff810f4fd0>] mmap_region+0x26f/0x466
> >         [<ffffffff810f545b>] do_mmap_pgoff+0x294/0x2ee
> >         [<ffffffff810f55a9>] sys_mmap_pgoff+0xf4/0x12f
> >         [<ffffffff8103d1f2>] sys_mmap+0x1d/0x1f
> >         [<ffffffff816940a2>] system_call_fastpath+0x16/0x1b
> > 
> >  other info that might help us debug this:
> > 
> >   Possible unsafe locking scenario:
> > 
> >         CPU0                    CPU1
> >         ----                    ----
> >    lock(&mm->mmap_sem);
> >                                 lock(&sb->s_type->i_mutex_key#12);
> >                                 lock(&mm->mmap_sem);
> >    lock(&sb->s_type->i_mutex_key#12);
> > 
> >   *** DEADLOCK ***
> > 
> >  1 lock held by shared/1568:
> >   #0:  (&mm->mmap_sem){++++++}, at: [<ffffffff810f5589>] sys_mmap_pgoff+0xd4/0x12f
> > 
> >  stack backtrace:
> >  Pid: 1568, comm: shared Not tainted 3.3.0-rc4+ #190
> >  Call Trace:
> >   [<ffffffff81688bf9>] print_circular_bug+0x1f8/0x209
> >   [<ffffffff8109f40a>] __lock_acquire+0xa6c/0xd60
> >   [<ffffffff8110e7b6>] ? files_lglock_local_lock_cpu+0x61/0x61
> >   [<ffffffff811efa0f>] ? hugetlbfs_file_mmap+0x7d/0x108
> >   [<ffffffff8109fb8f>] lock_acquire+0xd5/0xfa
> >   [<ffffffff811efa0f>] ? hugetlbfs_file_mmap+0x7d/0x108
> > 
> 
> Why have these lockdep warnings started coming out now - was the VFS
> changed to newly take i_mutex somewhere in the directory handling?

I'm not sure that they're new warnings. My patch (linked to below) may
have just gave folks a false hope that their nagging lockdep problems
are over.

> 
> 
> Sigh.  Was lockdep_annotate_inode_mutex_key() sufficiently
> self-explanatory to justify leaving it undocumented?
> 
> <goes off and reads e096d0c7e2e>
> 
> OK, the patch looks correct given the explanation in e096d0c7e2e, but
> I'd like to understand why it becomes necessary only now.
> 
> > NOTE: This patch also require 
> > http://thread.gmane.org/gmane.linux.file-systems/58795/focus=59565
> > to remove the lockdep warning
> 
> And that patch has been basically ignored.

Al commented on it here:

https://lkml.org/lkml/2012/2/16/518

He said that while my patch is correct, taking i_mutex inside mmap_sem
is still wrong.

Tyler

> 
> Sigh.  I guess I'll grab both patches, but I'm not confident in doing
> so without an overall explanation of what is happening here.
> 
>