linux-kernel - Re: [PATCH -v2 3/4] locking: Introduce smp_mb__after

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Date:   Thu, 3 Aug 2017 16:28:20 +0100
From:   Will Deacon <will.deacon@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     torvalds@...ux-foundation.org, oleg@...hat.com,
        paulmck@...ux.vnet.ibm.com, benh@...nel.crashing.org,
        mpe@...erman.id.au, npiggin@...il.com,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        stern@...land.harvard.edu
Subject: Re: [PATCH -v2 3/4] locking: Introduce smp_mb__after_spinlock().

On Wed, Aug 02, 2017 at 01:38:40PM +0200, Peter Zijlstra wrote:
> Since its inception, our understanding of ACQUIRE, esp. as applied to
> spinlocks, has changed somewhat. Also, I wonder if, with a simple
> change, we cannot make it provide more.
> 
> The problem with the comment is that the STORE done by spin_lock isn't
> itself ordered by the ACQUIRE, and therefore a later LOAD can pass over
> it and cross with any prior STORE, rendering the default WMB
> insufficient (pointed out by Alan).
> 
> Now, this is only really a problem on PowerPC and ARM64, both of
> which already defined smp_mb__before_spinlock() as a smp_mb().
> 
> At the same time, we can get a much stronger construct if we place
> that same barrier _inside_ the spin_lock(). In that case we upgrade
> the RCpc spinlock to an RCsc.  That would make all schedule() calls
> fully transitive against one another.
> 
> Cc: Alan Stern <stern@...land.harvard.edu>
> Cc: Nicholas Piggin <npiggin@...il.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Michael Ellerman <mpe@...erman.id.au>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> Cc: Paul McKenney <paulmck@...ux.vnet.ibm.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  arch/arm64/include/asm/spinlock.h   |    2 ++
>  arch/powerpc/include/asm/spinlock.h |    3 +++
>  include/linux/atomic.h              |    3 +++
>  include/linux/spinlock.h            |   36 ++++++++++++++++++++++++++++++++++++
>  kernel/sched/core.c                 |    4 ++--
>  5 files changed, 46 insertions(+), 2 deletions(-)
> 
> --- a/arch/arm64/include/asm/spinlock.h
> +++ b/arch/arm64/include/asm/spinlock.h
> @@ -367,5 +367,7 @@ static inline int arch_read_trylock(arch
>   * smp_mb__before_spinlock() can restore the required ordering.
>   */
>  #define smp_mb__before_spinlock()	smp_mb()
> +/* See include/linux/spinlock.h */
> +#define smp_mb__after_spinlock()	smp_mb()
>  
>  #endif /* __ASM_SPINLOCK_H */

Acked-by: Will Deacon <will.deacon@....com>

Will