linux-kernel - Re: [RFC PATCH 1/2] Add unlocked version of igrab.

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Date:	Mon, 28 Mar 2011 17:39:13 +1300
From:	Ryan Mallon <ryan@...ewatersys.com>
To:	Matthew Wilcox <matthew@....cx>
CC:	viro@...iv.linux.org.uk, dchinner@...hat.com,
	Trond.Myklebust@...app.com, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-nfs@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] Add unlocked version of igrab.

On 03/28/2011 03:54 PM, Matthew Wilcox wrote:
> On Mon, Mar 28, 2011 at 02:56:00PM +1300, Ryan Mallon wrote:
>> Commit 250df6ed274d767da844a5d9f05720b804240197 "fs: protect
>> inode->i_state with inode->i_lock" changes igrab to acquire inode->i_lock,
>> however some callees, notably nfs_inode_add_request, already hold the lock
>> when calling igrab.
> 
> I think a better solution to your problem is to notice that this is
> called in the context of doing a write to an inode.  That means we
> must already have a reference count on this inode, so it can't possibly
> be in I_FREEING or I_WILL_FREE.  That means we can just call __iget()
> instead ... except that __iget isn't exported to modules.

Ah, okay. Thanks for the hint.

A few other locations that I can see that call igrab with inode->i_lock
held are:

  fs/ceph/snap.c::ceph_queue_cap_snap
  fs/ceph/addr.c::ceph_set_page_dirty
  fs/nfs/nfs4state.c::nfs4_get_open_state

There may be some more cases where the locking is less obvious. I don't
know enough about the filesystem code to say whether each of those can
skip the (I_FREEING | I_WILL_FREE) check, or whether the correct
approach is to modify the filesystems themselves so that they do not
hold i_lock when calling igrab (i.e. rework to use a different outer lock)?

If the correct approach is to use __iget or __igrab then I can prepare a
patch for this. In the case of __iget, should it just be marked
EXPORT_SYMBOL and added to include/linux/fs.h?

~Ryan

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