linux-kernel - Re: [PATCH] staging: greybus: Convert timers to use timer

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Date:   Tue, 24 Oct 2017 14:30:10 +0100
From:   Bryan O'Donoghue <pure.logic@...us-software.ie>
To:     Kees Cook <keescook@...omium.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Johan Hovold <johan@...nel.org>, Alex Elder <elder@...nel.org>,
        greybus-dev@...ts.linaro.org, devel@...verdev.osuosl.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] staging: greybus: Convert timers to use timer_setup()

On 24/10/17 14:14, Kees Cook wrote:
> On Tue, Oct 24, 2017 at 5:52 AM, Bryan O'Donoghue
> <pure.logic@...us-software.ie> wrote:
>> On 24/10/17 13:47, Kees Cook wrote:
>>>
>>> On Tue, Oct 24, 2017 at 2:40 AM, Bryan O'Donoghue
>>> <pure.logic@...us-software.ie> wrote:
>>>>
>>>> On 24/10/17 10:35, Bryan O'Donoghue wrote:
>>>>>
>>>>>
>>>>> On 24/10/17 09:25, Kees Cook wrote:
>>>>>>
>>>>>>
>>>>>> In preparation for unconditionally passing the struct timer_list
>>>>>> pointer
>>>>>> to
>>>>>> all timer callbacks, switch to using the new timer_setup() and
>>>>>> from_timer()
>>>>>> to pass the timer pointer explicitly.
>>>>>>
>>>>>> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>>>>>> Cc: "Bryan O'Donoghue" <pure.logic@...us-software.ie>
>>>>>> Cc: Johan Hovold <johan@...nel.org>
>>>>>> Cc: Alex Elder <elder@...nel.org>
>>>>>> Cc: greybus-dev@...ts.linaro.org
>>>>>> Cc: devel@...verdev.osuosl.org
>>>>>> Signed-off-by: Kees Cook <keescook@...omium.org>
>>>>>> ---
>>>>>>     drivers/staging/greybus/loopback.c  | 14 ++++----------
>>>>>>     drivers/staging/greybus/operation.c |  7 +++----
>>>>>>     2 files changed, 7 insertions(+), 14 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/staging/greybus/loopback.c
>>>>>> b/drivers/staging/greybus/loopback.c
>>>>>> index 08e255884206..045aaf81113a 100644
>>>>>> --- a/drivers/staging/greybus/loopback.c
>>>>>> +++ b/drivers/staging/greybus/loopback.c
>>>>>> @@ -572,16 +572,11 @@ static void
>>>>>> gb_loopback_async_operation_work(struct
>>>>>> work_struct *work)
>>>>>>         gb_loopback_async_operation_put(op_async);
>>>>>>     }
>>>>>> -static void gb_loopback_async_operation_timeout(unsigned long data)
>>>>>> +static void gb_loopback_async_operation_timeout(struct timer_list *t)
>>>>>>     {
>>>>>> -    struct gb_loopback_async_operation *op_async;
>>>>>> -    u16 id = data;
>>>>>> +    struct gb_loopback_async_operation *op_async =
>>>>>> +        from_timer(op_async, t, timer);
>>>>>> -    op_async = gb_loopback_operation_find(id);
>>>>>> -    if (!op_async) {
>>>>>> -        pr_err("operation %d not found - time out ?\n", id);
>>>>>> -        return;
>>>>>> -    }
>>>>>
>>>>>
>>>>>
>>>>> Hi Kees, you need to add
>>>>>
>>>>>        gb_loopback_async_operation_get(op_async); when dropping the
>>>>> gb_loopback_operation_find() call here.
>>>>
>>>>
>>>>
>>>> Actually:
>>>>
>>>>           spin_lock_irqsave(&gb_dev.lock, flags);
>>>>           gb_loopback_async_operation_get(op_async);
>>>>           spin_unlock_irqrestore(&gb_dev.lock, flags);
>>>
>>> Shouldn't the get/put follow the lifetime of the timer running
>>> instead? It shouldnt' be possible to free the op_async while the timer
>>> is still pending/running.
>>
>> The timeout timer runs for an operation that never completed but on the
>> regular "everything is good" path you end up doing
>> del_timer_sync(&op_async->timer); so the timer doesn't run.
> 
> As far as I can tell, the get/put is already happening external to the timer:
> 
> gb_loopback_async_operation():
> ...
>          op_async = kzalloc(sizeof(*op_async), GFP_KERNEL);
> ...
>          INIT_WORK(&op_async->work, gb_loopback_async_operation_work);
>          kref_init(&op_async->kref);
> ...
>          op_async->pending = true;
> ...
>          ret = gb_operation_request_send(operation,
>                                          gb_loopback_async_operation_callback,
>                                          0,
>                                          GFP_KERNEL);
> ...
>          timer_setup(&op_async->timer, gb_loopback_async_operation_timeout, 0);
>          op_async->timer.expires = jiffies + gb->jiffy_timeout;
>          add_timer(&op_async->timer);
> 
> 
> gb_loopback_async_operation_callback(): (run by operation)
> ...
>                  op_async->pending = false;
>                  del_timer_sync(&op_async->timer);
>                  gb_loopback_async_operation_put(op_async);
> 
> 
> gb_loopback_async_operation_work(): (scheduled by timer)
> ...
>                  op_async->pending = false;
>                  gb_loopback_async_operation_put(op_async);
> 


> (Doing a get/put in the timer callback itself shouldn't be happening:
> it must already be pinned in memory for the callback to run.)

Both gb_loopback_async_operation_callback() and 
(gb_loopback_async_operation_timeout || 
gb_loopback_async_operation_work()) can run in parallel so the get in 
the timer callback means we can let the timer stuff free-run w/r to 
gb_loopback_async_operation_callback() - take a reference indicating we 
still want that object and then let gb_loopback_async_operation_work() run.

---
bod