From: "Rafael J. Wysocki" <rjw@sisk.pl>
To: Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: Async suspend-resume patch w/ rwsems (was: Re: [GIT PULL] PM updates for 2.6.33)
Date: Wed, 9 Dec 2009 00:24:24 +0100
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Cc: Alan Stern <stern@rowland.harvard.edu>, Zhang Rui <rui.zhang@intel.com>,
       LKML <linux-kernel@vger.kernel.org>,
       ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
       pm list <linux-pm@lists.linux-foundation.org>
References: <Pine.LNX.4.44L0.0912072225390.4179-100000@netrider.rowland.org> <200912082303.12758.rjw@sisk.pl> <200912082355.00711.rjw@sisk.pl>
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On Tuesday 08 December 2009, Rafael J. Wysocki wrote:
> On Tuesday 08 December 2009, Rafael J. Wysocki wrote:
> > On Tuesday 08 December 2009, Rafael J. Wysocki wrote:
> > > On Tuesday 08 December 2009, Linus Torvalds wrote:
> > > > 
> > > > 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_.
> > > 
> > > Understood (I think).
> > > 
> > > Let's try it, then.  Below is the resume patch based on my previous one in this
> > > thread (I have only verified that it builds).
> > 
> > Ah, I need to check if dev->parent is not NULL before trying to lock it, but
> > apart from this it doesn't break things at least.
> 
> For completness, below is the full async suspend/resume patch with rwlocks,
> that has been (very slightly) tested and doesn't seem to break things.
> 
> [Note to Alan: lockdep doesn't seem to complain about the not annotated nested
> locks.]

BTW, I can easily change it so that it uses completions for synchronization,
but I'm not sure if that's worth spending time on, so please let me know.

Rafael
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