From: Arnd Bergmann <arnd@arndb.de>
To: Binoy Jayan <binoy.jayan@linaro.org>
Cc: Doug Ledford <dledford@redhat.com>, Sean Hefty <sean.hefty@intel.com>,
        Hal Rosenstock <hal.rosenstock@gmail.com>, linux-rdma@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 6/8] IB/hns: Replace counting semaphore event_sem with wait_event
Date: Tue, 25 Oct 2016 14:28:57 +0200
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On Tuesday, October 25, 2016 5:31:57 PM CEST Binoy Jayan wrote:
>  static int __hns_roce_cmd_mbox_wait(struct hns_roce_dev *hr_dev, u64 in_param,
>                                     u64 out_param, unsigned long in_modifier,
> @@ -198,11 +218,12 @@ static int __hns_roce_cmd_mbox_wait(struct hns_roce_dev *hr_dev, u64 in_param,
>         struct hns_roce_cmdq *cmd = &hr_dev->cmd;
>         struct device *dev = &hr_dev->pdev->dev;
>         struct hns_roce_cmd_context *context;
> -       int ret = 0;
> +       int orig_free_head, ret = 0;
> +
> +       wait_event(cmd->wq, (orig_free_head = atomic_free_node(cmd, -1)) != -1);
>  
>         spin_lock(&cmd->context_lock);
> -       WARN_ON(cmd->free_head < 0);
> -       context = &cmd->context[cmd->free_head];
> +       context = &cmd->context[orig_free_head];
>         context->token += cmd->token_mask + 1;
>         cmd->free_head = context->next;
>         spin_unlock(&cmd->context_lock);
> 

You get the lock in atomic_free_node() and then again right after that.
Why not combine the two and only take the lock inside of that
function that returns a context?

	Arnd