linux-kernel - Re: A potential bug in drivers/iio/light/opt3001.ko

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Date:   Mon, 5 Sep 2016 17:15:30 +0300
From:   Pavel Andrianov <andrianov@...ras.ru>
To:     Jonathan Cameron <jic23@...nel.org>
Cc:     Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alexander Koch <mail@...xanderkoch.net>,
        Vaishali Thakkar <vaishali.thakkar@...cle.com>,
        ldv-project@...uxtesting.org,
        Andreas Dannenberg <dannenberg@...com>
Subject: Re: A potential bug in drivers/iio/light/opt3001.ko

03.09.2016 19:38, Jonathan Cameron пишет:
> On 31/08/16 11:23, Pavel Andrianov wrote:
>> Hi!
>>
>> There is a bug in drivers/iio/light/opt3001.ko. Regard such case:
>>
>> Thread 1                             Thread 2
>> -> opt3001_read_raw
>>   -> mutex_lock(&opt->lock)
>>   -> opt3001_get_lux()
>>     ..
>>     ->i2c_smbus_write_word_swapped()
>>             Now an interrupt comes
>>                                      -> opt3001_irq
>>                                        -> mutex_lock(&opt->lock)
>>
>> This is a deadlock, as the flag ok_to_ignore_lock has not been set yet.
> Good find.  Will need reordering to set the ok_to_ignore_lock first.
> Whether it ever actually happens will depend on just how long that EOC
> interrupt takes to happen.  Still it's a theoretical problem with
> a fairly simple fix so let's fix it.
>>
>> Regard another case:
>>
>> Thread 1                             Thread 2
>> -> opt3001_read_raw
>>   -> mutex_lock(&opt->lock)
>>   -> opt3001_get_lux()
>>     ..
>>     -> i2c_smbus_write_word_swapped()
>>     opt->ok_to_ignore_lock = true;
>>             Now an interrupt comes
>>                                      -> opt3001_irq
>>                                        ..
>>                                        opt->result_ready = true
>>                                        wake_up()
>>      opt->result_ready = false;
>>      wait_event_timeout()
>>
>> In this case the first thread misses the result and waits until timeout expires.
>>
> Agreed - looks like some reordering is needed here as well.
>
> Jonathan
>

In opt3001_get_lux has a comment, that i2c_smbus_write_word_swapped 
(line 246) enables interrupt mechanism. If an interrupt can not arise 
before the function, the assignments to both of flags should be moved 
before i2c_smbus_write_word_swapped and this is the best fix for both of 
issues.
Do you know if my assumption is correct and interrupts are disabled 
before i2c_smbus_write_word_swapped call?
-- 
Pavel Andrianov
Linux Verification Center, ISPRAS
web: http://linuxtesting.org
e-mail: andrianov@...ras.ru