linux-kernel - Re: [PATCH] kernel:async function call:introduce async

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Date:	Wed, 13 May 2009 13:02:33 +0200
From:	Cornelia Huck <cornelia.huck@...ibm.com>
To:	tom.leiming@...il.com
Cc:	arjan@...radead.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, Ming Lei <tom.leiming@...il.com>
Subject: Re: [PATCH] kernel:async function call:introduce async_run

On Wed, 13 May 2009 08:33:49 +0800,
tom.leiming@...il.com wrote:

> From: Ming Lei <tom.leiming@...il.com>
> 
> In fact, some async function call does not need to check, async_run
> is right for this kind of function call. Also, async_run is very
> suitable to used to start a thread in atomic context to do somthing,
> for now there is no such kind of kernel api, eg. kthread_run
> can not be started in atomic context.

Could you rework your explanation a bit? If I understand correctly, you
want to introduce a way to queue an async thread for those callers that
don't want to synchronize on cookies.

> 
> This patch is againt my another patch:
>    kernel/async.c:introduce async_schedule*_atomic
> please review.
> 
> Signed-off-by: Ming Lei <tom.leiming@...il.com>
> ---
>  include/linux/async.h |    1 +
>  kernel/async.c        |   24 +++++++++++++++++++++++-
>  2 files changed, 24 insertions(+), 1 deletions(-)
> 
> diff --git a/include/linux/async.h b/include/linux/async.h
> index ede9849..5390572 100644
> --- a/include/linux/async.h
> +++ b/include/linux/async.h
> @@ -16,6 +16,7 @@
>  typedef u64 async_cookie_t;
>  typedef void (async_func_ptr) (void *data, async_cookie_t cookie);
> 
> +extern void async_run(async_func_ptr *ptr, void *data);
>  extern async_cookie_t async_schedule(async_func_ptr *ptr, void *data);
>  extern async_cookie_t async_schedule_domain(async_func_ptr *ptr, void *data,
>  					    struct list_head *list);
> diff --git a/kernel/async.c b/kernel/async.c
> index 6bf565b..9cc4670 100644
> --- a/kernel/async.c
> +++ b/kernel/async.c
> @@ -60,6 +60,7 @@ asynchronous and synchronous parts of the kernel.
> 
>  static async_cookie_t next_cookie = 1;
> 
> +#define MAX_COOKIE	(~0)
>  #define MAX_THREADS	256
>  #define MAX_WORK	32768
> 
> @@ -207,7 +208,10 @@ static async_cookie_t __async_schedule(async_func_ptr *ptr, void *data, \
>  	entry->running = running;
> 
>  	spin_lock_irqsave(&async_lock, flags);
> -	newcookie = entry->cookie = next_cookie++;
> +	if (atomic == 1)
> +		newcookie = entry->cookie = next_cookie++;
> +	else
> +		newcookie = entry->cookie = MAX_COOKIE;

This confuses me. Why do you change the behaviour for atomic == 0?

>  	list_add_tail(&entry->list, &async_pending);
>  	atomic_inc(&entry_count);
>  	spin_unlock_irqrestore(&async_lock, flags);
> @@ -216,6 +220,24 @@ static async_cookie_t __async_schedule(async_func_ptr *ptr, void *data, \
>  }
> 
>  /**
> + * async_run - schedule a function for asynchronous execution
> + * @ptr: function to execute asynchronously
> + * @data: data pointer to pass to the function
> + *
> + * Note:we do not allocate a cookie for this kind of aysnchronous
> + * function to decrease the wait time of async_synchronize_full().

But async_synchronize_full() still waits for list_empty(&async_running)
- so what does this buy us?

> + * In fact, some function does not need to check, async_run is right
> + * for this kind of function call. Also, async_run is very suitable to
> + * start a thread to do somthing in atomic context,for now there is no such
> + * kind of kernel api, eg. kthread_run can not be run in atomic context.

Hm...
"The purpose of this function is to offer a simple way to schedule an
asynchronous thread, especially from an atomic context."
Would that describe async_run() better?

Doesn't this function need a return code since queueing the async work
can fail?

> + */
> +void async_run(async_func_ptr *ptr, void *data)
> +{
> +	__async_schedule(ptr, data, &async_running, 2);

I don't like the overloading of the "atomic" value - if I causually
looked at the declaration of __async_schedule(), I'd think it would be
a kind of boolean value...

> +}
> +EXPORT_SYMBOL_GPL(async_run);
> +
> +/**
>   * async_schedule - schedule a function for asynchronous execution
>   * @ptr: function to execute asynchronously
>   * @data: data pointer to pass to the function


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