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Date: Mon, 6 Jun 2016 14:52:05 +1000 (AEST)
From: Michael Ellerman <mpe@...erman.id.au>
To: Boqun Feng <boqun.feng@...il.com>, linuxppc-dev@...ts.ozlabs.org,
linux-kernel@...r.kernel.org
Cc: Peter Zijlstra <peterz@...radead.org>,
Boqun Feng <boqun.feng@...il.com>,
Will Deacon <will.deacon@....com>,
Paul Mackerras <paulus@...ba.org>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [v2] powerpc: spinlock: Fix spin_unlock_wait()
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
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