Subject: Re: [v2] powerpc: spinlock: Fix spin_unlock_wait()

From: Boqun Feng <[email-masked]>
Date: Mon, 6 Jun 2016 12:59:59 +0800

On Mon, Jun 06, 2016 at 02:52:05PM +1000, Michael Ellerman wrote:
> On Fri, 2016-03-06 at 03:49:48 UTC, Boqun Feng wrote:
> > There is an ordering issue with spin_unlock_wait() on powerpc, because
> > the spin_lock primitive is an ACQUIRE and an ACQUIRE is only ordering
> > the load part of the operation with memory operations following it.
> 
> ...
> > diff --git a/arch/powerpc/include/asm/spinlock.h b/arch/powerpc/include/asm/spinlock.h
> > index 523673d7583c..2ed893662866 100644
> > --- a/arch/powerpc/include/asm/spinlock.h
> > +++ b/arch/powerpc/include/asm/spinlock.h
> > @@ -162,12 +181,23 @@ static inline void arch_spin_unlock(arch_spinlock_t *lock)
> >  	lock->slock = 0;
> >  }
> >  
> > -#ifdef CONFIG_PPC64
> > -extern void arch_spin_unlock_wait(arch_spinlock_t *lock);
> > -#else
> > -#define arch_spin_unlock_wait(lock) \
> > -	do { while (arch_spin_is_locked(lock)) cpu_relax(); } while (0)
> > -#endif
> > +static inline void arch_spin_unlock_wait(arch_spinlock_t *lock)
> > +{
> > +	smp_mb();
> > +
> > +	if (!arch_spin_is_locked_sync(lock))
> > +		goto out;
> > +
> > +	while (!arch_spin_value_unlocked(*lock)) {
> > +		HMT_low();
> > +		if (SHARED_PROCESSOR)
> > +			__spin_yield(lock);
> > +	}
> > +	HMT_medium();
> > +
> > +out:
> > +	smp_mb();
> > +}
> 
> I think this would actually be easier to follow if it was all just in one routine:
> 
> 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, 1) "\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();
> }
> 
> 
> Thoughts?
> 

Make sense. I admit that I sort of overdesigned by introducing
arch_spin_is_locked_sync().

This version is better, thank you!

Regards,
Boqun

> cheers