linux-kernel - Re: [PATCH v2 7/4] ipc: fine grained locking for semtimedop

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Date:	Wed, 6 Mar 2013 17:57:56 -0500
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Davidlohr Bueso <davidlohr.bueso@...com>,
	Emmanuel Benisty <benisty.e@...il.com>,
	"Vinod, Chegu" <chegu_vinod@...com>,
	"Low, Jason" <jason.low2@...com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>, aquini@...hat.com,
	Michel Lespinasse <walken@...gle.com>,
	Ingo Molnar <mingo@...nel.org>,
	Larry Woodman <lwoodman@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>, hhuang@...hat.com
Subject: Re: [PATCH v2 7/4] ipc: fine grained locking for semtimedop

On Wed, Mar 6, 2013 at 5:15 PM, Rik van Riel <riel@...hat.com> wrote:
>
> If the call is a semop manipulating just one semaphore in
> an array with multiple semaphores, the read/write lock for
> the semaphore array is taken in shared (read) mode, and the
> individual semaphore's lock is taken.

You know, we do something like this already elsewhere, and I think we
do it slightly better. See the mm_take_all_locks() logic in mm/mmap.c.

The optimal strategy if you have many items, and the *common* case is
that you need just one lock, is actually to only take that single lock
for the common case. No top-level lock at all.

Then, for the complex (but unlikely) case, you take a *separate*
top-level lock, and then you take the lower-level locks one by one,
while testing first if you already hold them (using a separate data
structure that is protected by the top-level lock).

This way, the common case only needs that single lock that protects
just that particular data structure.

That said, judging by your numbers, your read-write lock seems to work
fine too, even though I'd worry about cacheline ping-pong (but not
contention) on the readers. So it doesn't seem optimal, but it sure as
hell seems better than what we do now ;)

                Linus
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