On Fri, May 04, 2018 at 01:54:14PM +1000, NeilBrown wrote: > If the sequence: > obj = rhashtable_walk_next(iter); > rhashtable_walk_stop(iter); > rhashtable_remove_fast(ht, &obj->head, params); > rhashtable_walk_start(iter); > > races with another thread inserting or removing > an object on the same hash chain, a subsequent > rhashtable_walk_next() is not guaranteed to get the "next" > object. It is possible that an object could be > repeated, or missed. > > This can be made more reliable by keeping the objects in a hash chain > sorted by memory address. A subsequent rhashtable_walk_next() > call can reliably find the correct position in the list, and thus > find the 'next' object. > > It is not possible (certainly not so easy) to achieve this with an > rhltable as keeping the hash chain in order is not so easy. When the > first object with a given key is removed, it is replaced in the chain > with the next object with the same key, and the address of that > object may not be correctly ordered. > No current user of rhltable_walk_enter() calls > rhashtable_walk_start() more than once, so no current code > could benefit from a more reliable walk of rhltables. > > This patch only attempts to improve walks for rhashtables. > - a new object is always inserted after the last object with a > smaller address, or at the start > - when rhashtable_walk_start() is called, it records that 'p' is not > 'safe', meaning that it cannot be dereferenced. The revalidation > that was previously done here is moved to rhashtable_walk_next() > - when rhashtable_walk_next() is called while p is not NULL and not > safe, it walks the chain looking for the first object with an > address greater than p and returns that. If there is none, it moves > to the next hash chain. > > Signed-off-by: NeilBrown <ne...@suse.com>
I'm a bit torn on this. On the hand this is definitely an improvement over the status quo. On the other this does not work on rhltable and we do have a way of fixing it for both rhashtable and rhltable. Cheers, -- Email: Herbert Xu <herb...@gondor.apana.org.au> Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
