Re: Async resume patch (was: Re: [GIT PULL] PM updates for 2.6.33)

Linus Torvalds
2009-12-08T21:04:02Z

On Tue, 8 Dec 2009, Rafael J. Wysocki wrote:
>
> Anyway, if we use an rwsem, it won't be checkable from interrupt context just
> as well.

You can't do a lock() from an interrupt, but the unlocks should be
irq-safe.

> Suppose we use rwsem and during suspend each child uses a down_read() on a
> parent and then the parent uses down_write() on itself. What if, whatever the
> reason, the parent is a bit early and does the down_write() before one of the
> children has a chance to do the down_read()? Aren't we toast?

We're toast, but we're toast for a totally unrealted reason: it means that
you tried to resume a child before a parent, which would be a major bug to
begin with.

Look, I even wrote out the comments, so let me repeat the code one more
time.

- suspend time calling:
// This won't block, because we suspend nodes before parents
down_read(node->parent->lock);
// Do the part that may block asynchronously
async_schedule(do_usb_node_suspend, node);

- resume time calling:
// This won't block, because we resume parents before children,
// and the children will take the read lock.
down_write(leaf->lock);
// Do the blocking part asynchronously
async_schedule(usb_node_resume, leaf);

See? So when we take the parent lock for suspend, we are guaranteed to do
so _before_ the parent node itself suspends. And conversely, when we take
the parent lock (asynchronously) for resume, we're guaranteed to do that
_after_ the parent node has done its own down_write.

And that all depends on just one trivial thing; that the suspend and
resume is called in the right order (children first vs parent first
respectively). And that is such a _major_ correctness issue that if that
isn't correct, your suspend isn't going to work _anyway_.

Linus