linux-kernel - Re: [PATCH 2/4] barrier.h: Move smp_mb__after_unlock

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Message-Id: <20160828134321.GC19706@linux.vnet.ibm.com>
Date:   Sun, 28 Aug 2016 06:43:22 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Manfred Spraul <manfred@...orfullife.com>
Cc:     benh@...nel.crashing.org, Ingo Molnar <mingo@...e.hu>,
        Boqun Feng <boqun.feng@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, 1vier1@....de,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH 2/4] barrier.h: Move smp_mb__after_unlock_lock to
 barrier.h

On Sun, Aug 28, 2016 at 01:56:14PM +0200, Manfred Spraul wrote:
> spin_unlock() + spin_lock() together do not form a full memory barrier:
> 
>   a=1;
>   spin_unlock(&b);
>   spin_lock(&c);
> + smp_mb__after_unlock_lock();
>   d=1;

Better would be s/d=1/r1=d/ above.

Then another process doing this:

	d=1
	smp_mb()
	r2=a

might have the after-the-dust-settles outcome of r1==0&&r2==0.

The advantage of this scenario is that it can happen on real hardware.

> 
> Without the smp_mb__after_unlock_lock(), other CPUs can observe the
> write to d without seeing the write to a.
> 
> Signed-off-by: Manfred Spraul <manfred@...orfullife.com>

With the upgraded commit log, I am OK with the patch below.
However, others will probably want to see at least one use of
smp_mb__after_unlock_lock() outside of RCU.

							Thanx, Paul

> ---
>  include/asm-generic/barrier.h | 16 ++++++++++++++++
>  kernel/rcu/tree.h             | 12 ------------
>  2 files changed, 16 insertions(+), 12 deletions(-)
> 
> diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
> index fe297b5..9b4d28f 100644
> --- a/include/asm-generic/barrier.h
> +++ b/include/asm-generic/barrier.h
> @@ -244,6 +244,22 @@ do {									\
>  	smp_acquire__after_ctrl_dep();				\
>  	VAL;							\
>  })
> +
> +#ifndef smp_mb__after_unlock_lock
> +/*
> + * Place this after a lock-acquisition primitive to guarantee that
> + * an UNLOCK+LOCK pair act as a full barrier.  This guarantee applies
> + * if the UNLOCK and LOCK are executed by the same CPU or if the
> + * UNLOCK and LOCK operate on the same lock variable.
> + */
> +#ifdef CONFIG_PPC
> +#define smp_mb__after_unlock_lock()	smp_mb()  /* Full ordering for lock. */
> +#else /* #ifdef CONFIG_PPC */
> +#define smp_mb__after_unlock_lock()	do { } while (0)
> +#endif /* #else #ifdef CONFIG_PPC */
> +
> +#endif
> +
>  #endif
> 
>  #endif /* !__ASSEMBLY__ */
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index e99a523..a0cd9ab 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -687,18 +687,6 @@ static inline void rcu_nocb_q_lengths(struct rcu_data *rdp, long *ql, long *qll)
>  #endif /* #ifdef CONFIG_RCU_TRACE */
> 
>  /*
> - * Place this after a lock-acquisition primitive to guarantee that
> - * an UNLOCK+LOCK pair act as a full barrier.  This guarantee applies
> - * if the UNLOCK and LOCK are executed by the same CPU or if the
> - * UNLOCK and LOCK operate on the same lock variable.
> - */
> -#ifdef CONFIG_PPC
> -#define smp_mb__after_unlock_lock()	smp_mb()  /* Full ordering for lock. */
> -#else /* #ifdef CONFIG_PPC */
> -#define smp_mb__after_unlock_lock()	do { } while (0)
> -#endif /* #else #ifdef CONFIG_PPC */
> -
> -/*
>   * Wrappers for the rcu_node::lock acquire and release.
>   *
>   * Because the rcu_nodes form a tree, the tree traversal locking will observe
> -- 
> 2.5.5
>