Re: NFS: possible circular locking i_mutex <> mmap_sem

[Andrew Morton]

On Thu, 18 Jun 2009 10:32:08 +0800 Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:

> Hi,
> 
> I got a lockdep warning on a stress testing a nfsroot desktop.
> The bits puzzled me is how come the page_fault() happens in
> generic_file_buffered_write()?

write(fd, buf, count).  If the page at *buf isn't present, we take a
fault to instantiate that page so we can copy_to_user() into it.

> Thanks,
> Fengguang
> 
> [ 2638.515865] =======================================================
> [ 2638.519743] [ INFO: possible circular locking dependency detected ]
> [ 2638.519743] 2.6.30-rc8-mm1 #307
> [ 2638.519743] -------------------------------------------------------
> [ 2638.519743] firefox-bin/3399 is trying to acquire lock:
> [ 2638.519743]  (&mm->mmap_sem){++++++}, at: [<ffffffff81548471>] do_page_fault+0x301/0x330
> [ 2638.519743]
> [ 2638.519743] but task is already holding lock:
> [ 2638.519743]  (&sb->s_type->i_mutex_key#6){+.+.+.}, at: [<ffffffff810c2bd2>] generic_file_aio_write+0x52/0xd0
> [ 2638.519743]
> [ 2638.519743] which lock already depends on the new lock.
> [ 2638.519743]
> [ 2638.519743]
> [ 2638.519743] the existing dependency chain (in reverse order) is:
> [ 2638.519743]
> [ 2638.519743] -> #1 (&sb->s_type->i_mutex_key#6){+.+.+.}:
> [ 2638.519743]        [<ffffffff8107c066>] __lock_acquire+0x12b6/0x1b40
> [ 2638.519743]        [<ffffffff8107c9d1>] lock_acquire+0xe1/0x120
> [ 2638.519743]        [<ffffffff8154328e>] mutex_lock_nested+0x5e/0x390
> [ 2638.519743]        [<ffffffff811be15c>] nfs_revalidate_mapping+0xac/0x110    ==> takes i_mutex in nfs_invalidate_mapping()
> [ 2638.519743]        [<ffffffff811bba25>] nfs_file_mmap+0x55/0x80
> [ 2638.519743]        [<ffffffff810e2bb7>] mmap_region+0x427/0x600
> [ 2638.519743]        [<ffffffff810e305e>] do_mmap_pgoff+0x2ce/0x3f0
> [ 2638.519743]        [<ffffffff81010c26>] sys_mmap+0x106/0x130                 ==> takes mmap_sem
> [ 2638.519743]        [<ffffffff8100bf42>] system_call_fastpath+0x16/0x1b
> [ 2638.519743]        [<ffffffffffffffff>] 0xffffffffffffffff

This is the faulty code path.  mmap_sem is supposed to nest inside i_mutex.

> [ 2638.519743] -> #0 (&mm->mmap_sem){++++++}:
> [ 2638.519743]        [<ffffffff8107c206>] __lock_acquire+0x1456/0x1b40
> [ 2638.519743]        [<ffffffff8107c9d1>] lock_acquire+0xe1/0x120
> [ 2638.519743]        [<ffffffff81543d3b>] down_read+0x4b/0x80
> [ 2638.519743]        [<ffffffff81548471>] do_page_fault+0x301/0x330            ==> takes mmap_sem
> [ 2638.519743]        [<ffffffff81545b55>] page_fault+0x25/0x30
> [ 2638.519743]        [<ffffffff810c1970>] generic_file_buffered_write+0x100/0x320
> [ 2638.519743]        [<ffffffff810c202d>] __generic_file_aio_write_nolock+0x25d/0x450
> [ 2638.519743]        [<ffffffff810c2be9>] generic_file_aio_write+0x69/0xd0     ==> takes i_mutex
> [ 2638.519743]        [<ffffffff811bbd36>] nfs_file_write+0x136/0x230
> [ 2638.519743]        [<ffffffff810fc991>] do_sync_write+0xf1/0x140
> [ 2638.519743]        [<ffffffff810fd59b>] vfs_write+0xcb/0x1a0
> [ 2638.519743]        [<ffffffff810fd760>] sys_write+0x50/0x90
> [ 2638.519743]        [<ffffffff8100bf42>] system_call_fastpath+0x16/0x1b
> [ 2638.519743]        [<ffffffffffffffff>] 0xffffffffffffffff
> [ 2638.519743]

<digs around a bit>

afaict this was added by a fix-a-compile-warning patch!

: commit e1ebfd33be068ec933f8954060a499bd22ad6f69
: Author:     Trond Myklebust <Trond.Myklebust@xxxxxxxxxx>
: AuthorDate: Wed Mar 11 14:37:54 2009 -0400
: Commit:     Trond Myklebust <Trond.Myklebust@xxxxxxxxxx>
: CommitDate: Wed Mar 11 14:37:54 2009 -0400
: 
:     NFS: Kill the "defined but not used" compile error on nommu machines
:     
:     Bryan Wu reports that when compiling NFS on nommu machines he gets a
:     "defined but not used" error on nfs_file_mmap().
:     
:     The easiest fix is simply to get rid of the special casing in NFS, and
:     just always call generic_file_mmap() to set up the file.
:     
:     Signed-off-by: Trond Myklebust <Trond.Myklebust@xxxxxxxxxx>
: 
: diff --git a/fs/nfs/file.c b/fs/nfs/file.c
: index 404c19c..1eab9c9 100644
: --- a/fs/nfs/file.c
: +++ b/fs/nfs/file.c
: @@ -64,11 +64,7 @@ const struct file_operations nfs_file_operations = {
:  	.write		= do_sync_write,
:  	.aio_read	= nfs_file_read,
:  	.aio_write	= nfs_file_write,
: -#ifdef CONFIG_MMU
:  	.mmap		= nfs_file_mmap,
: -#else
: -	.mmap		= generic_file_mmap,
: -#endif
:  	.open		= nfs_file_open,
:  	.flush		= nfs_file_flush,
:  	.release	= nfs_file_release,
: @@ -304,11 +300,13 @@ nfs_file_mmap(struct file * file, struct vm_area_struct * vma)
:  	dprintk("NFS: mmap(%s/%s)\n",
:  		dentry->d_parent->d_name.name, dentry->d_name.name);
:  
: -	status = nfs_revalidate_mapping(inode, file->f_mapping);
: +	/* Note: generic_file_mmap() returns ENOSYS on nommu systems
: +	 *       so we call that before revalidating the mapping
: +	 */
: +	status = generic_file_mmap(file, vma);
:  	if (!status) {
:  		vma->vm_ops = &nfs_file_vm_ops;
: -		vma->vm_flags |= VM_CAN_NONLINEAR;
: -		file_accessed(file);
: +		status = nfs_revalidate_mapping(inode, file->f_mapping);
:  	}
:  	return status;
:  }

It's odd/worrying that this problem sat in linux-next for a month
and nobody noticed.
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