netdev - Re: [Question] bluetooth/{bnep,cmtp,hidp}: memory barriers

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Date:   Tue, 14 Aug 2018 12:26:58 +0800
From:   JeffyChen <jeffy.chen@...k-chips.com>
To:     Brian Norris <computersforpeace@...il.com>,
        Andrea Parri <andrea.parri@...rulasolutions.com>
CC:     Marcel Holtmann <marcel@...tmann.org>,
        Johan Hedberg <johan.hedberg@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        linux-bluetooth@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Brian Norris <briannorris@...omium.org>,
        AL Yu-Chen Cho <acho@...e.com>
Subject: Re: [Question] bluetooth/{bnep,cmtp,hidp}: memory barriers

Hi guys,

Thanks for your mails, and sorry for the late response..

On 08/14/2018 07:18 AM, Brian Norris wrote:
>
> commit 5da8e47d849d3d37b14129f038782a095b9ad049
> Author: Jeffy Chen<jeffy.chen@...k-chips.com>
> Date:   Tue Jun 27 17:34:44 2017 +0800
>
>      Bluetooth: hidp: fix possible might sleep error in hidp_session_thread
>
> that*some*  kind of barrier was stuck in there simply as a response to
> comments like this, that were going away:
>
> -                *
> -                * Note: set_current_state() performs any necessary
> -                * memory-barriers for us.
>                   */
> -               set_current_state(TASK_INTERRUPTIBLE);
>
> +               /* Ensure session->terminate is updated */
> +               smp_mb__before_atomic();
>
>
> It was probably an attempt to fill in the gap for the
> set_current_state() (and comment) which was being removed. I believe
> Jeffy originally added more barriers in other places, but I convinced
> him not to.

right, i was trying to avoid losing memory-barriers when removing 
set_current_state and changing wake_up_process to wake_up_interruptible.

and checking these code again, it's true the smp_mb__before_atomic 
before atomic_read is not needed, the smp_mb after 
atomic_inc(&session->terminate) should be enough.

and as Brian point out, there's already an smp_store_mb at the end of 
wait_woken, i agree we can remove all the 
smp_mb__{before,after}_atomic() i wrongly added :)

>
> I have to say, I'm not really up-to-speed on the use of manual barriers
> in Linux (it's much preferable when they're wrapped into higher-level
> data structures already), but I believe the main intention here is to
> ensure that any change to 'terminate' that happened during the previous
> "wait_woken()" would be visible to our atomic_read().
>
> Looking into wait_woken(), I'm feeling like none of these additional
> barriers are necessary at all. I believe wait_woken() handles the
> visibility issues we care about (that if we were woken for termination,
> we'll see the terminating condition).