linux-kernel - Re: [RFC][PATCH] spinlock: Kill spin_unlock

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Date:	Thu, 06 Jan 2011 10:32:33 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Nick Piggin <npiggin@...nel.dk>,
	Chris Mason <chris.mason@...cle.com>,
	Frank Rowand <frank.rowand@...sony.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <efault@....de>,
	Oleg Nesterov <oleg@...hat.com>, Paul Turner <pjt@...gle.com>,
	Jens Axboe <axboe@...nel.dk>,
	Yong Zhang <yong.zhang0@...il.com>,
	linux-kernel@...r.kernel.org,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Linux-Arch <linux-arch@...r.kernel.org>,
	Jeff Garzik <jeff@...zik.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [RFC][PATCH] spinlock: Kill spin_unlock_wait()

On Wed, 2011-01-05 at 11:43 -0800, Linus Torvalds wrote:
> On Wed, Jan 5, 2011 at 11:14 AM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> >
> > There appear to be only two callsites of said horror, one in the exit
> > path and one in ata-eh, neither appear to be performance critical so I
> > replaced them with a simple lock-unlock sequence.
> 
> Again, WHY?
> 
> What's the problem with the current code? Instead of generating ugly
> patches to change it, and instead of removing it, just say what the
> PROBLEM is.

Well, I don't care about the primitive anymore, and Nick had some
reasonable arguments on why its not a good primitive to have. So in a
brief moment I decided to see what it would take to make it go away.

Apparently you don't like it, I'm fine with that, consider the patch
discarded.

> Some simple helper functions to extract the tail/head part of the
> ticket lock to make the comparisons understandable,

Jeremy has a number of pending patches making things more pretty. If you
wish I can revisit this once that work hits your tree.

  http://lkml.org/lkml/2010/11/16/479

He makes the thing looks like:

+#if (CONFIG_NR_CPUS < 256)
+typedef u8  __ticket_t;
+#else
+typedef u16 __ticket_t;
+#endif
+
+#define TICKET_SHIFT   (sizeof(__ticket_t) * 8)
+#define TICKET_MASK    ((__ticket_t)((1 << TICKET_SHIFT) - 1))
+
 typedef struct arch_spinlock {
+       union {
+               unsigned int slock;
+               struct __raw_tickets {
+                       __ticket_t head, tail;
+               } tickets;
+       };
 } arch_spinlock_t;

>  together with
> always accessing the lock with the proper ACCESS_ONCE() would have
> made your previous patch acceptable.

I'm still not quite seeing where I was missing an ACCESS_ONCE(), the
second loop had a cpu_relax() in, which is a compiler barrier so it
forces a reload that way.

>  But you ignored that feedback,
> and instead you now want to do a "let's just remove it entirely patch"
> that is even worse.

My locking improved and became a lot more obvious by not using the
primitive, so for the work I was doing not using it seemed the better
solution.

And as said, this was inspired by Nick's comments and it was a quick
edit to see what it would take.
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