linux-kernel - Re: [Patch v3] rwsem: fix rwsem_is

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Date:	Wed, 07 Oct 2009 13:19:11 +0100
From:	David Howells <dhowells@...hat.com>
To:	Amerigo Wang <amwang@...hat.com>
Cc:	dhowells@...hat.com, linux-kernel@...r.kernel.org,
	Ben Woodard <bwoodard@...l.gov>, akpm@...ux-foundation.org,
	Brian Behlendorf <behlendorf1@...l.gov>
Subject: Re: [Patch v3] rwsem: fix rwsem_is_locked() bugs

Amerigo Wang <amwang@...hat.com> wrote:

>  static inline int rwsem_is_locked(struct rw_semaphore *sem)
>  {
> -	return (sem->activity != 0);
> +	int ret;
> +
> +	if (spin_trylock_irq(&sem->wait_lock)) {
> +		ret = !(list_empty(&sem->wait_list) && sem->activity == 0);
> +		spin_unlock_irq(&sem->wait_lock);
> +		return ret;
> +	}
> +	return 1;
>  }

Yep...  This seems a reasonable approach, though I contend that if you're
holding the spinlock, then sem->wait_list _must_ be empty if sem->activity is
0 - so that half of the test is redundant.

sem->activity == 0 and sem->wait_list not being empty is a transitional state
that can only occur in ups and downgrades whilst they hold the spinlock.

> diff --git a/lib/rwsem-spinlock.c b/lib/rwsem-spinlock.c
> index 9df3ca5..234d83f 100644
> --- a/lib/rwsem-spinlock.c
> +++ b/lib/rwsem-spinlock.c
> @@ -78,7 +78,12 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
>  
>  	/* grant an infinite number of read locks to the front of the queue */
>   dont_wake_writers:
> -	woken = 0;
> +	/*
> +	 * we increase ->activity just to make rwsem_is_locked() happy,
> +	 * to avoid potential cache line ping-pong, we don't do this
> +	 * within the following loop.
> +	 */
> +	woken = sem->activity++;
>  	while (waiter->flags & RWSEM_WAITING_FOR_READ) {
>  		struct list_head *next = waiter->list.next;
>  
> @@ -94,7 +99,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
>  		waiter = list_entry(next, struct rwsem_waiter, list);
>  	}
>  
> -	sem->activity += woken;
> +	sem->activity = woken;
>  
>   out:
>  	return sem;

This change to __rwsem_do_wake() is all unnecessary - you're defending against
the test of sem->activity by rwsem_is_locked() - but that now happens with the
spinlock held.

David
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