Re: [PATCH RFC 1/2] qrwlock: A queue read/write lock implementation

[Waiman Long <waiman.long@hp.com>]

On 07/19/2013 05:11 PM, George Spelvin wrote:
>> What I have in mind is to have 2 separate rwlock initializers - one for
>> fair and one for reader-bias behavior. So the lock owners can decide
>> what behavior do they want with a one line change.
> That's definitely a nicer patch, if it will work.  I was imagining that,
> even for a single (type of) lock, only a few uses require reader bias
> (because they might be recursive, or are in an interrupt), but you'd
> want most read_lock sites to be fair.

Yes, fair rwlock will be the default.

> Deciding on a per-lock basis means that one potentially recursive call
> means you can't use fair queueing anywhere.
>
> I was hoping that the number of necessary unfair calls would
> be small enough that making the read_lock default fair and
> only marking the unfair call sites would be enough.
>
> But I don't really know until doing a survey of the calls.
I think so. The queue read/write lock, if merged, will be an optional 
feature for people to try out to see if they see any problem in any of 
the existing rwlock. So far, I didn't encounter any problem in my testing.

BTW, I also tried my version of the rwlock without the waiting queue. In 
high contention case, it performs slightly better than the 
__read_lock_failed changes suggested by Ingo at least for the 
reader-bias one. It is still not as good as the full version with the 
waiting queue. I should be able to provide more performance data next week.

Regards,
Longman