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

[Linus Torvalds]

On Sat, Sep 28, 2013 at 12:21 PM, Ingo Molnar <mingo@xxxxxxxxxx> 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).

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.

The old rwlock's really have been a disappointment - they are slower
than spinlocks, and seldom/never end up scaling any better.  Their
main advantage was literally the irq behavior - allowing readers to
happen without the expense of worrying about irq's.

              Linus
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