Re: [PATCH 3/6] ubifs: Use 64bit readdir cookies

From: Richard Weinberger
Date: Thu Dec 29 2016 - 11:36:44 EST


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.