On Wed, Dec 05, 2018 at 02:51:02PM +1100, 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 to take this approach 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. > I have not yet found any way to achieve the same stability > with rhltables, that doesn't have a major impact on lookup > or insert. No code currently in Linux would benefit from > such extra stability. > > With this patch: > - 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> > --- > > This is a resend of a patch that I sent back in July. I couldn't > applied then because it assumed another rhashtable patch which hadn't > landed yet - it now has.
I thought we had agreed to drop this because nobody needs it currently and it doesn't handle rhlist? 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
