linux-kernel - Re: [PATCH RFC ticketlock] Auto-queued ticketlock

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Message-ID: <CA+55aFyteA526MZHpzYTo1XoMx772FrRoqc_keODx1TfxDx6Fw@mail.gmail.com>
Date:	Tue, 11 Jun 2013 10:17:52 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:	Lai Jiangshan <eag0628@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Josh Triplett <josh@...htriplett.org>, niv@...ibm.com,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	David Howells <dhowells@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Darren Hart <darren@...art.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Silas Boyd-Wickizer <sbw@....edu>
Subject: Re: [PATCH RFC ticketlock] Auto-queued ticketlock

On Tue, Jun 11, 2013 at 9:48 AM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> 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.

                    Linus
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