linux-kernel - Re: [PATCH] rwsem: reduce spinlock contention in wakeup code path

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Message-ID: <CA+55aFymh9PP12Tt40HZB-JMv6Tan2aNJvnCScXpx3XrmddTAw@mail.gmail.com>
Date:	Sun, 29 Sep 2013 17:51:53 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	Ingo Molnar <mingo@...nel.org>, Waiman Long <Waiman.Long@...com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Alex Shi <alex.shi@...el.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Michel Lespinasse <walken@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH] rwsem: reduce spinlock contention in wakeup code path

On Sun, Sep 29, 2013 at 5:40 PM, Davidlohr Bueso <davidlohr@...com> wrote:
>
> Hmm, I'm getting the following at bootup:
>
>  May be due to missing lock nesting notation

Yes it is. And that reminds me of a problem I think we had with this
code: we had a possible case of the preemption counter nesting too
deeply. I forget the details, but it was something people worried
about.

That mm_take_all_locks() thing is really special, and I suspect that
if we go down this way we should just do a single preempt-disable and
then use the arch_write_lock() to avoid both the lockdep splat _and_
the preemption counter overflow.

                 Linus
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