Re: [PATCH RFC ticketlock] Auto-queued ticketlock

From: Paul E. McKenney
Date: Tue Jun 11 2013 - 13:33:14 EST

On Tue, Jun 11, 2013 at 10:17:52AM -0700, Linus Torvalds wrote:
> On Tue, Jun 11, 2013 at 9:48 AM, Paul E. McKenney
> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> >
> > Another approach is to permanently associate queues with each lock,
> > but that increases the size of the lock -- something that has raised
> > concerns in the past. But if adding 32 bytes to each ticketlock was OK,
> > this simplifies things quite a bit.
> Yeah, no. The spinlocks need to be small. We have them in
> size-conscious data structures like "struct dentry" and "struct page",
> and they really must not be bigger than an "int" in the non-debug
> case.
> In fact, I've occasionally thought about combining a spinlock with a
> refcounter if that could make things fit in 32 bits on smaller
> machines, because we also have ops like "atomic_dec_and_lock()" that
> could possibly be optimized if they fit in one word. That is probably
> not worth it, but spinlocks do need to remain small.

I was afraid of that. On the other hand, I guess that this means that
I sent out the correct patch of the two that I prepared. ;-)

Thanx, Paul

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