linux-kernel - Re: [PATCH v2 03/13] rwsem: move rwsem_down_failed_common code into rwsem_down_{read,write}

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Date:	Thu, 28 Mar 2013 13:05:32 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	Alex Shi <alex.shi@...el.com>, Ingo Molnar <mingo@...nel.org>,
	David Howells <dhowells@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Yuanhan Liu <yuanhan.liu@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	Dave Chinner <david@...morbit.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 03/13] rwsem: move rwsem_down_failed_common code into
 rwsem_down_{read,write}_failed

On Fri, 2013-03-15 at 03:54 -0700, Michel Lespinasse wrote:
> Remove the rwsem_down_failed_common function and replace it with two
> identical copies of its code in rwsem_down_{read,write}_failed.
> 
> This is because we want to make different optimizations in
> rwsem_down_{read,write}_failed; we are adding this pure-duplication
> step as a separate commit in order to make it easier to check the
> following steps.
> 
> Signed-off-by: Michel Lespinasse <walken@...gle.com>
> 
> ---
>  lib/rwsem.c | 72 ++++++++++++++++++++++++++++++++++++++++++++++++-------------
>  1 file changed, 57 insertions(+), 15 deletions(-)
> 
> diff --git a/lib/rwsem.c b/lib/rwsem.c
> index 40636454cf3c..fb658af1c12c 100644
> --- a/lib/rwsem.c
> +++ b/lib/rwsem.c
> @@ -178,12 +178,12 @@ try_again_write:
>  }
>  
>  /*
> - * wait for a lock to be granted
> + * wait for the read lock to be granted
>   */
> -static struct rw_semaphore __sched *
> -rwsem_down_failed_common(struct rw_semaphore *sem,
> -			 enum rwsem_waiter_type type, signed long adjustment)
> +struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
>  {
> +	enum rwsem_waiter_type type = RWSEM_WAITING_FOR_READ;
> +	signed long adjustment = -RWSEM_ACTIVE_READ_BIAS;

Again, just my opinion (and I suspect you only did this because that's
what was here) but I think this should be:

+	long adjustment = -RWSEM_ACTIVE_READ_BIAS;

>  	struct rwsem_waiter waiter;
>  	struct task_struct *tsk = current;
>  	signed long count;

Same here.

> @@ -238,21 +238,63 @@ rwsem_down_failed_common(struct rw_semaphore *sem,
>  }
>  
>  /*
> - * wait for the read lock to be granted
> - */
> -struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
> -{
> -	return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_READ,
> -					-RWSEM_ACTIVE_READ_BIAS);
> -}
> -
> -/*
>   * wait for the write lock to be granted
>   */
>  struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
>  {
> -	return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_WRITE,
> -					-RWSEM_ACTIVE_WRITE_BIAS);
> +	enum rwsem_waiter_type type = RWSEM_WAITING_FOR_WRITE;
> +	signed long adjustment = -RWSEM_ACTIVE_WRITE_BIAS;

Same here.

> +	struct rwsem_waiter waiter;
> +	struct task_struct *tsk = current;
> +	signed long count;

And here.

> +
> +	/* set up my own style of waitqueue */
> +	waiter.task = tsk;
> +	waiter.type = type;
> +	get_task_struct(tsk);
> +
> +	raw_spin_lock_irq(&sem->wait_lock);
> +	if (list_empty(&sem->wait_list))
> +		adjustment += RWSEM_WAITING_BIAS;
> +	list_add_tail(&waiter.list, &sem->wait_list);
> +
> +	/* we're now waiting on the lock, but no longer actively locking */
> +	count = rwsem_atomic_update(adjustment, sem);
> +
> +	/* If there are no active locks, wake the front queued process(es) up.
> +	 *
> +	 * Alternatively, if we're called from a failed down_write(), there
> +	 * were already threads queued before us and there are no active
> +	 * writers, the lock must be read owned; so we try to wake any read
> +	 * locks that were queued ahead of us. */
> +	if (count == RWSEM_WAITING_BIAS)
> +		sem = __rwsem_do_wake(sem, RWSEM_WAKE_NO_ACTIVE);
> +	else if (count > RWSEM_WAITING_BIAS &&
> +		 adjustment == -RWSEM_ACTIVE_WRITE_BIAS)
> +		sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
> +
> +	raw_spin_unlock_irq(&sem->wait_lock);
> +
> +	/* wait to be given the lock */
> +	while (true) {
> +		set_task_state(tsk, TASK_UNINTERRUPTIBLE);
> +		if (!waiter.task)
> +			break;
> +
> +		raw_spin_lock_irq(&sem->wait_lock);
> +		/* Try to get the writer sem, may steal from the head writer: */
> +		if (type == RWSEM_WAITING_FOR_WRITE)
> +			if (try_get_writer_sem(sem, &waiter)) {
> +				raw_spin_unlock_irq(&sem->wait_lock);
> +				return sem;
> +			}
> +		raw_spin_unlock_irq(&sem->wait_lock);
> +		schedule();
> +	}
> +
> +	tsk->state = TASK_RUNNING;
> +
> +	return sem;
>  }
>  
>  /*


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