Re: [RFC][PATCH] spinlock: Kill spin_unlock_wait()

From: Peter Zijlstra
Date: Thu Jan 06 2011 - 04:43:01 EST

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@xxxxxxxxx> 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

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

> 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.

He makes the thing looks like:

+#if (CONFIG_NR_CPUS < 256)
+typedef u8 __ticket_t;
+typedef u16 __ticket_t;
+#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

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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