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 08:34:06 -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 8:14 AM, Waiman Long <waiman.long@...com> wrote:
>
> Other than the global tty_ldisc_lock, there is no other major
> bottleneck. I am not that worry about the tty_ldisc_lock bottleneck
> as real world applications probably won't have that many calls to
> set the tty driver.

I suspect the tty_ldisc_lock() could be made to go away if we care.
Making the ldisc be rcu-free'd, coupled with just optimistically
updating the count using "atomic_inc_not_zero()" and re-checking the
ldisc pointer afterwards should probably do it. But I agree that it
probably isn't worth it for any actual real load.

              Linus
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