linux-kernel - Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless update of refcount

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Date:	Tue, 3 Sep 2013 12:09:27 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Waiman Long <waiman.long@...com>
Cc:	Ingo Molnar <mingo@...nel.org>, Al Viro <viro@...iv.linux.org.uk>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Jeff Layton <jlayton@...hat.com>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andi Kleen <andi@...stfloor.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless
 update of refcount

On Tue, Sep 3, 2013 at 8:34 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> I suspect the tty_ldisc_lock() could be made to go away if we care.

Heh. I just pulled the tty patches from Greg, and the locking has
changed completely.

It may actually fix your AIM7 test-case, because while the global
spinlock remains (it got renamed to "tty_ldiscs_lock" - there's an
added "s"), the common operations now take the per-tty lock to get the
ldisc for that tty, rather than that global spinlock (which just
protects the actual ldisk array now).

That said, I don't know what AIM7 really ends up doing, but your
profile seems to have every access through tty_ldisc_[de]ref() that
now uses only the per-tty lock. Of course, how much that helps ends up
depending on whether AIM7 uses lots of tty's or just one shared one.

Anyway, it might be worth testing my current -git tree.

                  Linus
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