Re: [PATCH] rwsem: reduce spinlock contention in wakeup code path

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

On 09/28/2013 03:33 PM, Linus Torvalds wrote:
> On Sat, Sep 28, 2013 at 12:21 PM, Ingo Molnar<mingo@kernel.org>  wrote:
>> If we do that then I suspect the next step will be queued rwlocks :-/ The
>> current rwlock_t implementation is rather primitive by modern standards.
>> (We'd probably have killed rwlock_t long ago if not for the
>> tasklist_lock.)
> Yeah, I'm not happy about or rwlocks. That's one lock that currently
> is so broken that I think we could easily argue for making that one
> queued.
>
> Waiman had a qrwlock series that looked reasonable, and I think his
> later versions were drop-in replacements (ie they automatically just
> did the RightThing(tm) wrt interrupts taking a recursive read lock - I
> objected to the first versions that required that to be stated
> explicitly).

The latest version (v3) will allow recursive read lock in interrupt 
handlers.

> I think Waiman's patches (even the later ones) made the queued rwlocks
> be a side-by-side implementation with the old rwlocks, and I think
> that was just being unnecessarily careful. It might be useful for
> testing to have a config option to switch between the two, but we
> might as well go all the way.

It is not actually a side-by-side implementation. A user can choose 
either regular rwlock or the queue one, but never both by setting a 
configuration parameter. However, I now think that maybe we should do it 
the lockref way by pre-determining it on a per-architecture level 
without user visible configuration option.

-Longman