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

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Date:	Tue, 8 Dec 2009 13:04:02 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	Alan Stern <stern@...land.harvard.edu>,
	Zhang Rui <rui.zhang@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	pm list <linux-pm@...ts.linux-foundation.org>
Subject: Re: Async resume patch (was: Re: [GIT PULL] PM updates for 2.6.33)

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
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