linux-kernel - Re: [PATCH -v3 3/8] locking: Introduce smp_acquire__after_ctrl

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Date:	Wed, 1 Jun 2016 21:52:14 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	manfred@...orfullife.com, dave@...olabs.net,
	paulmck@...ux.vnet.ibm.com, will.deacon@....com,
	Waiman.Long@....com, tj@...nel.org, pablo@...filter.org,
	kaber@...sh.net, davem@...emloft.net, oleg@...hat.com,
	netfilter-devel@...r.kernel.org, sasha.levin@...cle.com,
	hofrat@...dl.org
Subject: Re: [PATCH -v3 3/8] locking: Introduce smp_acquire__after_ctrl_dep

On Tue, May 31, 2016 at 11:41:37AM +0200, Peter Zijlstra wrote:
[snip]
> @@ -260,16 +260,6 @@ static void sem_rcu_free(struct rcu_head
>  }
>  
>  /*
> - * spin_unlock_wait() and !spin_is_locked() are not memory barriers, they
> - * are only control barriers.
> - * The code must pair with spin_unlock(&sem->lock) or
> - * spin_unlock(&sem_perm.lock), thus just the control barrier is insufficient.
> - *
> - * smp_rmb() is sufficient, as writes cannot pass the control barrier.
> - */
> -#define ipc_smp_acquire__after_spin_is_unlocked()	smp_rmb()
> -
> -/*
>   * Wait until all currently ongoing simple ops have completed.
>   * Caller must own sem_perm.lock.
>   * New simple ops cannot start, because simple ops first check
> @@ -292,7 +282,7 @@ static void sem_wait_array(struct sem_ar
>  		sem = sma->sem_base + i;
>  		spin_unlock_wait(&sem->lock);
>  	}
> -	ipc_smp_acquire__after_spin_is_unlocked();
> +	smp_acquire__after_ctrl_dep();

I wonder whether we can kill this barrier after updating
spin_unlock_wait() to ACQUIRE?

Regards,
Boqun

>  }
>  
>  /*

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