Re: [PATCH 6/8] rhashtable: further improve stability of rhashtable_walk

NeilBrown
2018-05-05T21:51:03Z

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On Sat, May 05 2018, Herbert Xu wrote:

> 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 <neilb@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.

Do we? How could we fix it for both rhashtable and rhltable?

Thanks,
NeilBrown

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