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

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Message-ID: <Pine.LNX.4.44L0.0912081431490.3046-100000@iolanthe.rowland.org>
Date:	Tue, 8 Dec 2009 14:34:22 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	"Rafael J. Wysocki" <rjw@...k.pl>, 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, Linus Torvalds wrote:

> Side note: if this was a real lock, you'd also needed an smp_wmb() in the 
> 'wait_lock()' path after the atomic_inc(), to make sure that others see 
> the atomic lock was seen by other people before the suspend started. 
> 
> In your usage scenario, I don't think it would ever be noticeable, since 
> the other users are always going to start running from the same thread 
> that did the wait_lock(), so even if they run on other CPU's, we'll have 
> scheduled _to_ those other CPU's and done enough memory ordering to 
> guarantee that they will see the thing.
> 
> So it would be ok in this situation, simply because it acts as an 
> initializer and never sees any real SMP issues.

Yes.  I would have brought this up, but you made the point for me.

> But it's an example of how you now don't just depend on the locking 
> primitives themselves doing the right thing, you end up depending very 
> subtly on exactly how the lock is used.  The standard locks do have the 
> same kind of issue for initializers, but we avoid it elsewhere because 
> it's so risky.

No doubt there are other reasons why the "wait-lock" pattern doesn't 
get used enough to be noticed.

Alan Stern

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