linux-kernel - Re: [PATCH 6/8] IB/hns: Replace counting semaphore event

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Date:   Mon, 17 Oct 2016 22:29:57 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Binoy Jayan <binoy.jayan@...aro.org>
Cc:     Doug Ledford <dledford@...hat.com>,
        Sean Hefty <sean.hefty@...el.com>,
        Hal Rosenstock <hal.rosenstock@...il.com>,
        linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/8] IB/hns: Replace counting semaphore event_sem with wait condition

On Monday, October 17, 2016 10:01:00 PM CEST Binoy Jayan wrote:
> --- a/drivers/infiniband/hw/hns/hns_roce_cmd.c
> +++ b/drivers/infiniband/hw/hns/hns_roce_cmd.c
> @@ -248,10 +248,14 @@ static int hns_roce_cmd_mbox_wait(struct hns_roce_dev *hr_dev, u64 in_param,
>  {
>         int ret = 0;
>  
> -       down(&hr_dev->cmd.event_sem);
> +       wait_event(hr_dev->cmd.event_sem.wq,
> +                  atomic_add_unless(&hr_dev->cmd.event_sem.count, -1, 0));
> +
>         ret = __hns_roce_cmd_mbox_wait(hr_dev, in_param, out_param,
>                                        in_modifier, op_modifier, op, timeout);
> -       up(&hr_dev->cmd.event_sem);
> +
> +       if (atomic_inc_return(&hr_dev->cmd.event_sem.count) == 1)
> +               wake_up(&hr_dev->cmd.event_sem.wq);
>  
>         return ret;
>  }

This is the only interesting use of the event_sem that cares about
the counting and it protects the __hns_roce_cmd_mbox_wait() from being
entered too often. The count here is the number of size of the
hr_dev->cmd.context[] array.

However, that function already use a spinlock to protect that array
and pick the correct context. I think changing the inner function
to handle the case of 'no context available' by using a waitqueue
without counting anything would be a reasonable transformation
away from the semaphore.

	Arnd