Subject: Re: [PATCH v3] powerpc: spinlock: Fix spin_unlock_wait()

From: Michael Ellerman <[email-masked]>
Date: Mon, 06 Jun 2016 22:17:25 +1000

On Mon, 2016-06-06 at 13:56 +0200, Peter Zijlstra wrote:
> On Mon, Jun 06, 2016 at 09:42:20PM +1000, Michael Ellerman wrote:
> > +static inline void arch_spin_unlock_wait(arch_spinlock_t *lock)
> > +{
> > +	arch_spinlock_t lock_val;
> > +
> > +	smp_mb();
> > +
> > +	/*
> > +	 * Atomically load and store back the lock value (unchanged). This
> > +	 * ensures that our observation of the lock value is ordered with
> > +	 * respect to other lock operations.
> > +	 */
> > +	__asm__ __volatile__(
> > +"1:	" PPC_LWARX(%0, 0, %2, 0) "\n"
> > +"	stwcx. %0, 0, %2\n"
> > +"	bne- 1b\n"
> > +	: "=&r" (lock_val), "+m" (*lock)
> > +	: "r" (lock)
> > +	: "cr0", "xer");
> > +
> > +	if (arch_spin_value_unlocked(lock_val))
> > +		goto out;
> > +
> > +	while (!arch_spin_value_unlocked(*lock)) {
> > +		HMT_low();
> > +		if (SHARED_PROCESSOR)
> > +			__spin_yield(lock);
> > +	}
> > +	HMT_medium();
> > +
> > +out:
> > +	smp_mb();
> > +}
> 
> Why the move to in-line this implementation? It looks like a fairly big
> function.

I agree it's not pretty.

I just didn't think having it out-of-line made it easier to understand. The
previous version had:

  static inline void arch_spin_unlock_wait(arch_spinlock_t *lock)
  {
  	...
  	if (!arch_spin_is_locked_sync(lock))
  		goto out;

Then elsewhere:

  static inline bool arch_spin_is_locked_sync(arch_spinlock_t *lock)
  {
  	...
  	return !arch_spin_value_unlocked(tmp);
  }

So two negations and one routine called "locked" and one "unlocked", which just
didn't read well IMHO.

Another minor concern was that someone might be "clever" and call the _sync()
version manually (though hopefully we'd catch that in review).

I'm not beholden to v3 though if you hate it.

cheers