linux-kernel - Re: [PATCH 3/5] locking/rtmutex: Use acquire/release semantics

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Message-ID: <alpine.DEB.2.11.1509221732460.5606@nanos>
Date:	Tue, 22 Sep 2015 17:33:12 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Davidlohr Bueso <dave@...olabs.net>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Will Deacon <will.deacon@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 3/5] locking/rtmutex: Use acquire/release semantics

On Mon, 21 Sep 2015, Davidlohr Bueso wrote:
> As such, weakly ordered archs can benefit from more relaxed use
> of barriers when locking/unlocking.
> 
> Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
> ---
>  kernel/locking/rtmutex.c | 30 +++++++++++++++++++++---------
>  1 file changed, 21 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
> index 7781d80..226a629 100644
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -74,14 +74,23 @@ static void fixup_rt_mutex_waiters(struct rt_mutex *lock)
>   * set up.
>   */
>  #ifndef CONFIG_DEBUG_RT_MUTEXES
> -# define rt_mutex_cmpxchg(l,c,n)	(cmpxchg(&l->owner, c, n) == c)
> +# define rt_mutex_cmpxchg_relaxed(l,c,n) (cmpxchg_relaxed(&l->owner, c, n) == c)
> +# define rt_mutex_cmpxchg_acquire(l,c,n) (cmpxchg_acquire(&l->owner, c, n) == c)
> +# define rt_mutex_cmpxchg_release(l,c,n) (cmpxchg_release(&l->owner, c, n) == c)
> +
> +/*
> + * Callers must hold the ->wait_lock -- which is the whole purpose as we force
> + * all future threads that attempt to [Rmw] the lock to the slowpath. As such
> + * relaxed semantics suffice.
> + */
>  static inline void mark_rt_mutex_waiters(struct rt_mutex *lock)
>  {
>  	unsigned long owner, *p = (unsigned long *) &lock->owner;
>  
>  	do {
>  		owner = *p;
> -	} while (cmpxchg(p, owner, owner | RT_MUTEX_HAS_WAITERS) != owner);
> +	} while (cmpxchg_relaxed(p, owner,
> +				 owner | RT_MUTEX_HAS_WAITERS) != owner);
>  }
>  
>  /*
> @@ -121,11 +130,14 @@ static inline bool unlock_rt_mutex_safe(struct rt_mutex *lock)
>  	 *					lock(wait_lock);
>  	 *					acquire(lock);
>  	 */
> -	return rt_mutex_cmpxchg(lock, owner, NULL);
> +	return rt_mutex_cmpxchg_acquire(lock, owner, NULL);

Why is this acquire?

Thanks,

	tglx
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