linux-kernel - Re: [dm-devel] [PATCH 1/2] swait: export the symbols __prepare_to_swait and __finish_swait

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Date:   Thu, 23 Nov 2017 00:28:57 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     dm-devel@...hat.com, Mike Snitzer <msnitzer@...hat.com>,
        Daniel Wagner <daniel.wagner@...-carit.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [dm-devel] [PATCH 1/2] swait: export the symbols
 __prepare_to_swait and __finish_swait

Please run this past the swait authors.  It is supposed to be a simple
and self-contained API so I'd expect this patch to be seen critical.

You might be better off to just use the normal complex waitqueues if
you want to micro-optimize like this.

On Wed, Nov 22, 2017 at 09:35:36PM -0500, Mikulas Patocka wrote:
> In order to reduce locking overhead, I use the spinlock in
> swait_queue_head to protect not only the wait queue, but also the list of
> events. Consequently, I need to use unlocked functions __prepare_to_swait
> and __finish_swait. These functions are declared in the file
> include/linux/swait.h, but they are not exported, and so they are not
> useable from kernel modules.
> 
> Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>
> 
> ---
>  kernel/sched/swait.c |    2 ++
>  1 file changed, 2 insertions(+)
> 
> Index: linux-2.6/kernel/sched/swait.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched/swait.c
> +++ linux-2.6/kernel/sched/swait.c
> @@ -73,6 +73,7 @@ void __prepare_to_swait(struct swait_que
>  	if (list_empty(&wait->task_list))
>  		list_add(&wait->task_list, &q->task_list);
>  }
> +EXPORT_SYMBOL(__prepare_to_swait);
>  
>  void prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait, int state)
>  {
> @@ -102,6 +103,7 @@ void __finish_swait(struct swait_queue_h
>  	if (!list_empty(&wait->task_list))
>  		list_del_init(&wait->task_list);
>  }
> +EXPORT_SYMBOL(__finish_swait);
>  
>  void finish_swait(struct swait_queue_head *q, struct swait_queue *wait)
>  {
> 
> --
> dm-devel mailing list
> dm-devel@...hat.com
> https://www.redhat.com/mailman/listinfo/dm-devel
---end quoted text---

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