linux-kernel - Re: [PATCH 4/4 v4] rtc: add rtc-driver for HID sensors of type time

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Date:	Fri, 14 Dec 2012 17:33:07 +0100
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Alexander Holler <holler@...oftware.de>
CC:	linux-kernel@...r.kernel.org, linux-iio@...r.kernel.org,
	Jonathan Cameron <jic23@....ac.uk>, rtc-linux@...glegroups.com,
	Alessandro Zummo <a.zummo@...ertech.it>,
	srinivas pandruvada <srinivas.pandruvada@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 4/4 v4] rtc: add rtc-driver for HID sensors of type time

On 12/14/2012 04:24 PM, Alexander Holler wrote:
> Am 14.12.2012 15:34, schrieb Lars-Peter Clausen:
>> On 12/14/2012 03:29 PM, Alexander Holler wrote:
>>> Am 14.12.2012 15:15, schrieb Alexander Holler:
>>>> Am 14.12.2012 14:08, schrieb Alexander Holler:
>>>>> Am 14.12.2012 10:42, schrieb Lars-Peter Clausen:
>>>>
>>>>>> And another thing I've overlooked before:
>>>>>> wait_for_completion_interruptible_timeout can either return a positive
>>>>>> number when the completion was completed, 0 in case of an timeout, or a
>>>>>> negative error code in case it was interrupted. You need to handle all
>>>>>> three. E.g. something like this.
>>>>>>
>>>>>> ret = wait_for_completion_interruptible_timeout(...)
>>>>>> if (ret == 0)
>>>>>>     return -EIO;
>>>>>> if (ret < 0)
>>>>>>     return ret
>>>>>>
>>>>>
>>>>> Hmpf, the only working approach to use some in kernel functions really
>>>>> is to the read source yourself and don't trust anything else. :/
>>>>
>>>> Anyway, my approach doesn't work as it introduces a race condition:
>>>>
>>>>
>>>> /* get a report with all values through requesting one value */
>>>> sensor_hub_input_attr_get_raw_value(...)
>>>>
>>>> /* race if this task goes to slepp and the values were
>>>> received before it could call the below wait...
>>>>
>>>> /* wait for all values (event) */
>>>> if (!wait_for_completion_interruptible_timeout(...))
>>>>
>>>>
>>>> I'll have to look for a mechanism how to avoid that. So v5 might need
>>>> some time.
>>>
>>> Sorry for the noise. That INIT_COMPLETION() before the sensor...() does
>>> exactly that. So it's enough if I handle the different return situations of
>>> wait_for...().
>>>
>>> I will just use if(wait...()<=0) return -EIO.
>>>
>>
>> No, that's wrong. You should really return the error code returned by
>> wait_for_completion_interruptible_timeout(). This will make sure that
>> userspace restarts the syscall if necessary.
> 
> Sorry for my ignorance, but which reasons for interruption do exist
> which doesn't kill the userspace too? The error number -ESYSRESTART
> doesn't offer a hint.


Well I'm not an expert on this either, but as far as I know any signal the
process is listening on can cause an interruption. Most signals won't kill
the process though. More on the whole restart stuff:
http://lwn.net/Articles/528935/

- Lars

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