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Groups > linux.kernel > #1471367
| From | "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> |
|---|---|
| Newsgroups | linux.kernel |
| Subject | Re: [PATCH 2/4] barrier.h: Move smp_mb__after_unlock_lock to barrier.h |
| Date | 2016-08-28 15:50 +0200 |
| Message-ID | <sb8Df-2mx-15@gated-at.bofh.it> (permalink) |
| References | <sb6UO-1cS-7@gated-at.bofh.it> <sb6UO-1cS-9@gated-at.bofh.it> <sb6UO-1cS-11@gated-at.bofh.it> |
| Organization | linux.* mail to news gateway |
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@colorfullife.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
>
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[PATCH 0/4] Clarify/standardize memory barriers for lock/unlock Manfred Spraul <manfred@colorfullife.com> - 2016-08-28 14:00 +0200
[PATCH 1/4] spinlock: Document memory barrier rules Manfred Spraul <manfred@colorfullife.com> - 2016-08-28 14:00 +0200
[PATCH 2/4] barrier.h: Move smp_mb__after_unlock_lock to barrier.h Manfred Spraul <manfred@colorfullife.com> - 2016-08-28 14:00 +0200
Re: [PATCH 2/4] barrier.h: Move smp_mb__after_unlock_lock to barrier.h "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-08-28 15:50 +0200
[PATCH 2/4] barrier.h: Move smp_mb__after_unlock_lock to barrier.h Manfred Spraul <manfred@colorfullife.com> - 2016-08-28 18:40 +0200
Re: [PATCH 2/4] barrier.h: Move smp_mb__after_unlock_lock to barrier.h Manfred Spraul <manfred@colorfullife.com> - 2016-08-28 20:10 +0200
Re: [PATCH 1/4] spinlock: Document memory barrier rules Peter Zijlstra <peterz@infradead.org> - 2016-08-29 12:50 +0200
Re: [PATCH 1/4] spinlock: Document memory barrier rules Manfred Spraul <manfred@colorfullife.com> - 2016-08-29 15:00 +0200
Re: [PATCH 1/4] spinlock: Document memory barrier rules Peter Zijlstra <peterz@infradead.org> - 2016-08-29 15:50 +0200
Re: [PATCH 1/4] spinlock: Document memory barrier rules Manfred Spraul <manfred@colorfullife.com> - 2016-08-31 07:00 +0200
Re: [PATCH 1/4] spinlock: Document memory barrier rules Peter Zijlstra <peterz@infradead.org> - 2016-08-31 17:50 +0200
Re: [PATCH 1/4] spinlock: Document memory barrier rules Will Deacon <will.deacon@arm.com> - 2016-08-31 18:50 +0200
Re: [PATCH 1/4] spinlock: Document memory barrier rules Manfred Spraul <manfred@colorfullife.com> - 2016-08-31 20:40 +0200
Re: [PATCH 1/4] spinlock: Document memory barrier rules Peter Zijlstra <peterz@infradead.org> - 2016-09-01 10:50 +0200
Re: [PATCH 1/4] spinlock: Document memory barrier rules Manfred Spraul <manfred@colorfullife.com> - 2016-09-01 13:10 +0200
Re: [PATCH 1/4] spinlock: Document memory barrier rules Will Deacon <will.deacon@arm.com> - 2016-09-01 13:20 +0200
Re: [PATCH 1/4] spinlock: Document memory barrier rules Peter Zijlstra <peterz@infradead.org> - 2016-09-01 14:00 +0200
Re: [PATCH 1/4] spinlock: Document memory barrier rules Boqun Feng <boqun.feng@gmail.com> - 2016-09-01 16:10 +0200
Re: [PATCH 0/4] Clarify/standardize memory barriers for lock/unlock Peter Zijlstra <peterz@infradead.org> - 2016-08-29 13:00 +0200
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