linux-kernel - Re: [PATCH v2 2/8] IB/core: Replace semaphore sm

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Date:   Tue, 25 Oct 2016 17:48:09 +0200
From:   Jack Wang <xjtuwjp@...il.com>
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>,
        Arnd Bergmann <arnd@...db.de>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        Linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/8] IB/core: Replace semaphore sm_sem with completion

Hi Binoy,
2016-10-25 17:08 GMT+02:00 Binoy Jayan <binoy.jayan@...aro.org>:
> On 25 October 2016 at 18:13, Jack Wang <xjtuwjp@...il.com> wrote:
>> Hi Binoy,
>>
>> snip
>>>
>>>         port->ib_dev   = device;
>>>         port->port_num = port_num;
>>> -       sema_init(&port->sm_sem, 1);
>>> +       init_completion(&port->sm_comp);
>>> +       complete(&port->sm_comp);
>>
>> Why complete here?
>>
>>>         mutex_init(&port->file_mutex);
>>>         INIT_LIST_HEAD(&port->file_list);
>>>
>>> --
>> KR,
>> Jinpu
>
>
> Hi Jack,
>
> ib_umad_sm_open() calls wait_for_completion_interruptible() which comes before
> ib_umad_sm_close() that calls complete(). In the initial open() there
> will not be
> anybody to signal the completion, so the complete is called to mark
> the initial state.
> I am not sure if this is the right way to do it, though.
>
> -Binoy

>From Documentation/scheduler/completion.txt ,
"
117 This is not implying any temporal order on wait_for_completion() and the
118 call to complete() - if the call to complete() happened before the call
119 to wait_for_completion() then the waiting side simply will continue
120 immediately as all dependencies are satisfied if not it will block until
121 completion is signaled by complete().
"
In this case here, if sm_open/sm_close are paired, it should work.

KR
Jack