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

[Pavel Andrianov]

03.09.2016 19:38, Jonathan Cameron ÐÐÑÐÑ:
> 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@xxxxxxxxx