Re: [PATCH 0/5] xfs: more patches for 3.13

[Eric Sandeen <sandeen@sandeen.net>]

On 11/6/13, 7:57 PM, Dave Chinner wrote:
> On Wed, Nov 06, 2013 at 05:01:33PM -0600, Ben Myers wrote:
>> On Fri, Nov 01, 2013 at 03:27:15PM +1100, Dave Chinner wrote:
>>> Hi folks,
>>>
>>> The following series follows up the recently committed series of
>>> patches for 3.13. The first two patches are the remaining
>>> uncommitted patches from the previous series.
>>>
>>> The next two patches are tracing patches, one for AIL manipulations
>>> and the other for AGF and AGI read operations. Both of these were
>>> written during recent debugging sessions, and both proved useful so
>>> should be added to the menagerie of tracepoints we already have
>>> avaialble.
>>>
>>> The final patch is the increasing of the inode cluster size for v5
>>> filesystems. I'd like to get this into v5 filesystems for 3.13 so we
>>> get wider exposure of it ASAP so we have more data available to be
>>> able to make informed decisions about how to bring this back to v4
>>> filesystems in a safe and controlled manner.
>>
>> Applied 3 and 4.  I still don't understand why the locking on patch 2 is
>> correct.  Seems like the readers of i_version hold different locks than we do
>> when we log the inode.  Maybe Christoph can help me with that.
> 
> Readers don't need to hold a spinlock, and many don't. The spinlock
> is only there to prevent concurrent updates from "losing" an update
> due to races.  All modifications to XFS inodes occur via
> transactions, inodes are locked exclusively in transactions and
> hence we will never lose i_version updates due to races. Hence we
> don't need the spinlock during the update, either.

I'm not completely convinced that readers don't need to.  What happens when
we read in the middle of an update?  Especially when a 32-bit box reads the
64-bit value in the middle of an update?

NFS is the only reader we care about (right?)

I see a several paths to i_version reads in nfs; so far I'm finding locked reads:

<2 callers of nfs_refresh_inode_locked>
	spin_lock(&inode->i_lock);
	nfs_refresh_inode_locked
		nfs_update_inode
			nfs_wcc_update_inode
				(... && inode->i_version == fattr->pre_change_attr)
		...
		if (inode->i_version != fattr->change_attr) {
		...
		nfs_check_inode_attributes
			(... && inode->i_version != fattr->change_attr)

---

update_changeattr
	spin_lock(&dir->i_lock);
	if (!cinfo->atomic || cinfo->before != dir->i_version)

---

nfs_post_op_update_inode_force_wcc
	 spin_lock(&inode->i_lock);
	 fattr->pre_change_attr = inode->i_version;

---

I haven't audited everything but do you have an example of an unlocked
reader (which is relevant to xfs)?

-Eric

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