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

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 0/5] xfs: more patches for 3.13
From: Eric Sandeen <sandeen@xxxxxxxxxxx>
Date: Fri, 15 Nov 2013 11:55:20 -0600
Cc: Ben Myers <bpm@xxxxxxx>, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <528657BB.8090206@xxxxxxxxxxx>
References: <1383280040-21979-1-git-send-email-david@xxxxxxxxxxxxx> <20131106230133.GX1935@xxxxxxx> <20131107015706.GM6188@dastard> <5282D2D3.3040601@xxxxxxxxxxx> <20131114011610.GM6188@dastard> <528657BB.8090206@xxxxxxxxxxx>
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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

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