Date: Mon, 30 Sep 2013 12:44:08 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ingo Molnar <mingo@kernel.org>, Waiman Long <Waiman.Long@hp.com>,
        Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Rik van Riel <riel@redhat.com>,
        Peter Hurley <peter@hurleysoftware.com>,
        Davidlohr Bueso <davidlohr.bueso@hp.com>,
        Alex Shi <alex.shi@intel.com>, Tim Chen <tim.c.chen@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Matthew R Wilcox <matthew.r.wilcox@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Michel Lespinasse <walken@google.com>,
        Andi Kleen <andi@firstfloor.org>,
        "Chandramouleeswaran, Aswin" <aswin@hp.com>,
        "Norton, Scott J" <scott.norton@hp.com>
Subject: Re: [PATCH] rwsem: reduce spinlock contention in wakeup code path
Message-ID: <20130930104408.GW3081@twins.programming.kicks-ass.net>
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On Sat, Sep 28, 2013 at 12:33:36PM -0700, Linus Torvalds wrote:
> The old rwlock's really have been a disappointment - they are slower
> than spinlocks, and seldom/never end up scaling any better.  Their
> main advantage was literally the irq behavior - allowing readers to
> happen without the expense of worrying about irq's.

So in part that is fundamental to the whole rw-spinlock concept.

Typically lock hold times should be short for spinlock type locks and if
your hold times are short, the lock acquisition times are significant.

And a read acquisition is still a RMW operation on the lock, thus read
locks are still entirely bound by the cacheline transfer of the lock
itself.
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