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

[Michael Ellerman]

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?

cheers