On Wed, 2017-07-26 at 02:09 +0200, Florian Westphal wrote:
> switch to lockless lockup. write side now also increments sequence
> counter. On lookup, sample counter value and only take the lock
> if we did not find a match and the counter has changed.
>
> This avoids need to write to private area in normal (lookup) cases.
>
> Note that we take the non-blocking variant (raw_seqcount_begin), i.e.
> read side will not wait for writer to finish.
>
> If we did not find a result we will fall back to use of read-lock.
>
> The readlock is also used during dumps to ensure we get a consistent
> tree walk.
>
> Similar technique (rbtree+seqlock) was used by David Howells in rxrpc.
Please note that in commit b145425f269a17ed344d737f746b844dfac60c82
("inetpeer: remove AVL implementation in favor of RB tree")
I chose to also pass the sequence so that the lookup could abort.
I am not sure that during rb tree write operations, some nodes could be
left with some kind of loop.
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