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

Eric Sandeen
2013-11-15T17:55:29Z

On 11/15/13, 11:19 AM, Eric Sandeen wrote:
> On 11/13/13, 7:16 PM, Dave Chinner wrote:
>
>> That's client side, not server side, so that's the NFS client inode
>> it is locking, not the server side XFS inode.
>
> Ah, geez, you're right. (x3)
>
> <snip>
>
>> Server side, where i_version is pulled out of an XFS inode:
>>
>> $ git grep i_version fs/nfsd
>> fs/nfsd/nfs3xdr.c: fhp->fh_post_change = fhp->fh_dentry->d_inode->i_version;
>> fs/nfsd/nfs4xdr.c: write64(p, inode->i_version);
>> fs/nfsd/nfsfh.h: fhp->fh_pre_change = inode->i_version;
>> $
>>
>> the nfsfh.h hit is in fill_pre_wcc(), which appears to be called
>> under i_mutex but not i_lock. The xdr encoding functions don't
>> appear to be holding i_lock, and may be holding i_mutex, but I
>> haven't looked that far.
>
> I'm still not sure how . . .

ugh didn't mean to send this reply quite yet, sorry.

Not sure how we do an unlocked read on a 32-bit machine that doesn't potentially
get the wrong answer. I talked to Bruce about it a bit; nothing jumped out at
us. At worst (?) it seems that if you happened to race on a read at exactly
the 2^32'nd modification, you might go backwards.

As Bruce says, even if so, maybe "so rare we don't care?"

-Eric