Re: Async suspend-resume patch w/ completions (was: Re: Async suspend-resume patch w/ rwsems)

Linus Torvalds
2009-12-10T02:52:37Z

On Thu, 10 Dec 2009, Rafael J. Wysocki wrote:
>
> Completions it is, then.

What was so hard with the "Try the simple one first" to understand? You
had a simpler working patch, why are you making this more complex one
without ever having had any problems with the simpler one?

Btw, your 'atomic_set()' with errors is pure voodoo programming. That's
not how atomics work. They do SMP-atomic addition etc, the 'atomic_set()'
and 'atomic_read()' things are not in any way more atomic than any other
access.

They are meant for racy reads (atomic_read()) and for initializations
(atomic_set()), and the way you use them that 'atomic' part is entirely
pointless, because it really isn't anything different from an 'int',
except that it may be very very expensive on some architectures due to
hashed spinlocks etc.

So stop this overdesign thing. Start simple. If you _ever_ see real
problems, that's when you add stuff. As it is, any time you add
complexity, you just add bugs.

> +/**
> + * dpm_synchronize - Wait for PM callbacks of all devices to complete.
> + */
> +static void dpm_synchronize(void)
> +{
> + struct device *dev;
> +
> + async_synchronize_full();
> +
> + mutex_lock(&dpm_list_mtx);
> + list_for_each_entry(dev, &dpm_list, power.entry)
> + INIT_COMPLETION(dev->power.completion);
> + mutex_unlock(&dpm_list_mtx);
> +}

And this, for example, is pretty disgusting. Not only is that
INIT_COMPLETION purely brought on by the whole problem with completions
(they are fundamentally one-shot, but you want to use them over and over
so you need to re-initialize them: a nice lock wouldn't have that problem
to begin with), but the comment isn't even accurate. Sure, it waits for
any async jobs, but that's the _least_ of what the function actually does,
so the comment is actively misleading, isn't it?

Linus