Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
> 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.
I see.
Ok, I will spin a v2 and will pass the sequence too, thanks Eric.
If we have to abort on seqretry anyway then I can also use
read_seqcount_begin to force readers to wait until writer is done, so I
will change that as well.
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