Re: [PATCH RFC ticketlock] Auto-queued ticketlock

From: Linus Torvalds
Date: Tue Jun 11 2013 - 13:17:58 EST

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

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.

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