On Sun, Mar 01, 2015 at 02:04:53PM +0100, Thorsten Bschorr wrote:
> Hi David,
>
> thanks for your feedback on my first patch, I wasn't aware of checkpatch.pl.
>
> Initially, I had just if-ed the usage of family-data, which did not
> look that nice. I was referring to this proof-of-concept workaround in
> my initial bug report.
>
> The patch I've submitted is different from my proof-of-concept
> workaround. Not unlocking the bus before returning clearly is an
> error, I did not extensively test this patch.
>
>
> > or just increment it while sleeping, which is when it's needed, which
> > also looks simpler.
> >
> > if (external_power) {
> > + int refcnt;
> > mutex_unlock(&dev->bus_mutex);
> >
> > + /* prevent the slave from going away */
> > + atomic_inc(&sl->refcnt);
> > sleep_rem = msleep_interruptible(tm);
> > + refcnt = w1_unref_slave(sl);
> > - if (sleep_rem != 0)
> > + if (sleep_rem != 0 || !refcnt)
> > return -EINTR;
> >
> > i =
> > mutex_lock_interruptible(&dev->bus_mutex);
> > if (i != 0)
> > return i;
> > } else if (!w1_strong_pullup) {
>
>
> I like this better than my workaround-patch.
>
> One thought occurred to me when looking at this proposal: wouldn't it
> be even better to increase sl->refcnt before unlocking the mutex?
> I was asking myself if it is possible that the current thread gets
> suspended between mutex_unlock(&dev->bus_mutex); and
> atomic_inc(&sl->refcnt); thus leaving another thread the change to
> unref the device?
> (I'm not that familiar with linux scheduling, so my assumption might be void.)
You are correct, it would be a race condition if it doesn't increment
the refcnt before unlocking the mutex, and it should get the mutex
before unref. Here's an updated version, I haven't even tried to
compile it.
What do you think Evgeniy?
if (external_power) {
int refcnt;
/* prevent the slave from going away in sleep */
atomic_inc(&sl->refcnt);
mutex_unlock(&dev->bus_mutex);
sleep_rem = msleep_interruptible(tm);
if (sleep_rem != 0) {
w1_unref_slave(sl);
return -EINTR;
}
i = mutex_lock_interruptible(&dev->bus_mutex);
refcnt = w1_unref_slave(sl);
if (i != 0) {
/* failed to lock */
return i;
}
if (!refcnt)
/* got lock, but slave went away */
mutex_unlock(&dev->bus_mutex);
return -EINTR;
}
} else if (!w1_strong_pullup) {
--
David Fries <da...@fries.net> PGP pub CB1EE8F0
http://fries.net/~david/
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