Re: [PATCH] NFSDv4: use export cache flushtime for changeid on V4ROOT objects.

[NeilBrown <neilb@suse.com>]

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On Tue, Jan 31 2017, J. Bruce Fields wrote:

> On Tue, Jan 31, 2017 at 09:28:37AM +1100, NeilBrown wrote:
>> On Mon, Jan 30 2017, J. Bruce Fields wrote:
>> 
>> > On Mon, Jan 30, 2017 at 05:17:00PM +1100, NeilBrown wrote:
>> >> 
>> >> If you change the set of filesystems that are exported, then
>> >> the contents of various directories in the NFSv4 pseudo-root
>> >> is likely to change.  However the change-id of those
>> >> directories is currently tied to the underlying directory,
>> >> so the clinet may not see the changes in a timely fashion.
>> >
>> > Oh, good catch.
>> >
>> >> This patch changes the change-id number to be derived from the
>> >> "flush_time" of the export cache.  Whenever any changes are
>> >> made to the set of exported filesystems, this flush_time is
>> >> updated.  The result is that clients see changes to the set
>> >> of exported filesystems much more quickly, often immediately.
>> >
>> > And, a clever solution, as usual....
>> >
>> > I wonder if it's completely right yet, though.  Off the top of my head:
>> > can't the client see the new flush time before it sees the new contents?
>> > If so, a client that caches both during that window could cache the old
>> > contents indefinitely.
>> 
>> uhm....
>> Yes, it could see the new flush time before it sees the new contents.
>> When it sees that new flush time (i.e. new change attribute), it will
>> invalidate its cached contents and ask for the contents again.
>
> The problem comes if it's still possible for the client to read (and
> cache) the old contents at this point, in which case the client's cache
> will incorrectly associate old contents with new change attribute.

I agree with this.

>
>> It will then definitely get new contents.
>
> So the problem with changing change attribute before contents is:
>
> 	- client retrieves old contents and new attribute, caches.
> 	- client revalidates cache at an arbitrarily later time, sees
> 	  it's still the new attribute, continues caching old contents.
>
> So usually I believe you want the two changes--contents and change
> attribute--to be atomic or, if that's not possible, for them to be
> changed in that order.

I believe that setting ->flush_time atomically effects both changes.

>
> I haven't thought through how that applies to this case, but I think it
> should be possible if in-progress rpc's hold references to objects in
> the flushed cache?

How would it do that?
In NFSv4 'READDIR' and 'GETATTR' are separate operations.
If the client sends READDIR and then GETATTR, it must not assume that
the change number in the GETATTR reply implies anything about the
READDIR reply.
But it (presumably) sends them in the order other, so if GETATTR gets a
new change number, then when nfsd4_encode_dirent_fattr() calls
nfsd_crossmnt() it will find the changed to the exports table, though it
may need to wait for an upcall to complete.

You are right to be cautious, but I think ->flush_time effectively
provides the needed atomicity.

Thanks,
NeilBrown

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