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

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Message-ID: <61728463-8bd8-c68e-ef4a-7a3e0ddd7899@kernel.org>
Date:   Sat, 3 Sep 2016 17:38:44 +0100
From:   Jonathan Cameron <jic23@...nel.org>
To:     Pavel Andrianov <andrianov@...ras.ru>
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

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