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 <ne...@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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