Re: [PATCH 6/6] rhashtable: add rhashtable_walk_prev()

[NeilBrown <neilb@suse.com>]

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On Wed, Apr 18 2018, Herbert Xu wrote:

> On Wed, Apr 18, 2018 at 04:47:02PM +1000, NeilBrown wrote:
>> rhashtable_walk_prev() returns the object returned by
>> the previous rhashtable_walk_next(), providing it is still in the
>> table (or was during this grace period).
>> This works even if rhashtable_walk_stop() and rhashtable_talk_start()
>> have been called since the last rhashtable_walk_next().
>> 
>> If there have been no calls to rhashtable_walk_next(), or if the
>> object is gone from the table, then NULL is returned.
>> 
>> This can usefully be used in a seq_file ->start() function.
>> If the pos is the same as was returned by the last ->next() call,
>> then rhashtable_walk_prev() can be used to re-establish the
>> current location in the table.  If it returns NULL, then
>> rhashtable_walk_next() should be used.
>> 
>> Signed-off-by: NeilBrown <neilb@suse.com>
>
> Can you explain the need for this function and its difference
> from the existing rhashtable_walk_peek?

The need is essentially the same as the need for
rhashtable_walk_peek().  The difference is that the actual behaviour can
be documented briefly (and so understood easily) without enumerating
multiple special cases.
rhashtable_walk_peek() is much the same as
 rhashtable_walk_prev() ?: rhashtable_walk_next()

The need arises when you need to "stop" a walk and then restart at the
same object, not the next one. i.e. the last object returned before the
"stop" wasn't acted on.
This happens with seq_file if the buffer space for ->show() is not
sufficient to format an object.  In the case, seq_file will stop() the
iteration, make more space available (either by flushing or by
reallocing) and will start again at the same location.
If the seq_file client stored the rhashtable_iter in the seq_file
private data, it can use rhasthable_walk_prev() to recover its position
if that object is still in the hash table.  If it isn't still present,
rhashtable_walk_next() can be used to get the next one.  In some cases
it can be useful for the client to know whether it got the previous one
or not.

Thanks,
NeilBrown

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