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

[NeilBrown]

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@xxxxxxxx>
>
> 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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