Subject: Re: [PATCH 3/6] ubifs: Use 64bit readdir cookies
To: "J. Bruce Fields" <bfields@fieldses.org>
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From: Richard Weinberger <richard@nod.at>
Message-ID: <224abf2f-c820-5df1-6292-42c1e2d47933@nod.at>
Date: Thu, 29 Dec 2016 17:36:35 +0100
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Bruce,

On 29.12.2016 17:15, J. Bruce Fields wrote:
> On Thu, Dec 29, 2016 at 04:49:54PM +0100, Richard Weinberger wrote:
>> Bruce,
>>
>> On 29.12.2016 16:34, J. Bruce Fields wrote:
>>>> That way UBIFS can provide a 64bit readdir() cookie which is required for NFS3.
>>>
>>> Sounds good.  And if a matching entry isn't found (as in the case of a
>>> concurrent unlink), what happens?  The answer must be the same as for
>>> ext4, but I've forgotten the details....  I guess it must find the next
>>> highest cookie (thinking of the cookie as a 64-bit integer of some kind)
>>> that exists in the directory.  And that must be the same order that
>>> readdir normally returns entries in.
>>
>> If a 64bit cookie is not found, the lookup function returns -ENOENT.
>> In UBIFS we cannot just select a higher or lower key (cookie in this case),
>> since it is the B-tree key and would point to a completely different
>> entry.
>>
>> So, in case of a concurrent unlink() one would succeed and one fail with
>> -ENOENT. Unless I miss something that seems okay to me.
> 
> Unlink takes (parent directory, name), not a directory cookie.
> 
> The problem is concurrent unlink and nfs readdir.  So:
> 
> 	NFS server returns readdir result with cookie X
> 
> 	Somebody unlinks the entry at X.
> 
> 	NFS server gets readdir request with cookie X.
> 
> Then the NFS client will get a spurious -ENOENT.

Ah yes. Sorry I misunderstood your question.
UBIFS readdir() address this already, if you ask it to readdir()
from pos X and X is not present it will jump to the next best entry X'.
UBIFS does so since ever.

Thanks,
//richard