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 21:30:14 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 Tuesday 08 December 2009, Alan Stern wrote:
> On Tue, 8 Dec 2009, Rafael J. Wysocki wrote:
> 
> > 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?
> > 
> > Do we need any direct protection against that or does it just work itself out
> > in a way I just don't see right now?
> 
> That's not the way it should be done.  Linus had children taking their
> parents' locks during suspend, which is simple but leads to
> difficulties.
> 
> Instead, the PM core should do a down_write() on each device before
> starting the device's async suspend routine, and an up_write() when the
> routine finishes.  Parents should, at the start of their async routine,
> do down_read() on each of their children plus whatever other devices
> they need to wait for.  The core can do the waiting for children part 
> and the driver's suspend routine can handle any other waiting.
> 
> This is a little more awkward because it requires the parent to iterate 
> through its children.

I can live with that.

> But it does solve the off-tree dependency  problem for suspends.

That's a plus, but I still think we're trying to create a barrier-alike
mechanism using lock.

There's one more possibility to consider, though.  What if we use a completion
instead of the flag + wait queue?  It surely is a standard synchronization
mechanism and it seems it might work here.

Rafael
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