linux-kernel - Re: [PATCH] rwsem: Support optimistic spinning

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Date:	Mon, 28 Apr 2014 09:52:05 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Alex Shi <alex.shi@...aro.org>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E.McKenney" <paulmck@...ux.vnet.ibm.com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rwsem: Support optimistic spinning

On Tue, Apr 22, 2014 at 03:19:26PM -0700, Davidlohr Bueso wrote:
> ---
>  include/linux/rwsem.h       |   9 +-
>  kernel/locking/rwsem-xadd.c | 213 +++++++++++++++++++++++++++++++++++++++-----
>  kernel/locking/rwsem.c      |  31 ++++++-
>  3 files changed, 231 insertions(+), 22 deletions(-)

rwsem-spinlock.c doesn't need changes?

> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> index cfff143..a911dbf 100644
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -12,6 +12,27 @@
>  
>  #include <linux/atomic.h>
>  
> +#ifdef CONFIG_SMP
> +static inline void rwsem_set_owner(struct rw_semaphore *sem)
> +{
> +	sem->owner = current;
> +}
> +
> +static inline void rwsem_clear_owner(struct rw_semaphore *sem)
> +{
> +	sem->owner = NULL;
> +}
> +
> +#else
> +static inline void rwsem_set_owner(struct rw_semaphore *sem)
> +{
> +}
> +
> +static inline void rwsem_clear_owner(struct rw_semaphore *sem)
> +{
> +}
> +#endif
> +
>  /*
>   * lock for reading
>   */
> @@ -48,6 +69,7 @@ void __sched down_write(struct rw_semaphore *sem)
>  	rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
>  
>  	LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
> +	rwsem_set_owner(sem);
>  }
>  
>  EXPORT_SYMBOL(down_write);
> @@ -59,8 +81,11 @@ int down_write_trylock(struct rw_semaphore *sem)
>  {
>  	int ret = __down_write_trylock(sem);
>  
> -	if (ret == 1)
> +	if (ret == 1) {
>  		rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_);
> +		rwsem_set_owner(sem);
> +	}
> +
>  	return ret;
>  }

So first acquire lock, then set owner.

> @@ -86,6 +111,7 @@ void up_write(struct rw_semaphore *sem)
>  	rwsem_release(&sem->dep_map, 1, _RET_IP_);
>  
>  	__up_write(sem);
> +	rwsem_clear_owner(sem);
>  }
>  
>  EXPORT_SYMBOL(up_write);
> @@ -100,6 +126,7 @@ void downgrade_write(struct rw_semaphore *sem)
>  	 * dependency.
>  	 */
>  	__downgrade_write(sem);
> +	rwsem_clear_owner(sem);
>  }

But here you first release and then clear owner; this is buggy. The
moment you release another acquire can happen and your clear will clear
the new owner, not us.

Or am I missing something obvious here?
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