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

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Message-Id: <200912082044.52098.rjw@sisk.pl>
Date:	Tue, 8 Dec 2009 20:44:52 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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 Tuesday 08 December 2009, Linus Torvalds wrote:
> 
> On Tue, 8 Dec 2009, Rafael J. Wysocki wrote:
> > 
> > The wait queue plus the op_complete flag combo plays the role of the locking
> > in the Linus' picture
> 
> Please just use the lock. Don't make up your own locking crap. Really. 
> 
> Your patch is horrible. Exactly because your locking is horribly 
> mis-designed. You can't say things are complete from an interrupt, for 
> example, since you made it some random bitfield, which has unknown 
> characteristics (ie non-atomic read-modify-write etc).

I didn't assume anyone would check it from an interrupt, because I didn't see
a point.  In fact I didn't assume anyone except for the PM core would check it.
In case this assumption is wrong, it can be easily put under the dev->sem
that we take anyway before calling the bus type (etc.) callbacks.

Anyway, if we use an rwsem, it won't be checkable from interrupt context just
as well.

> The fact is, any time anybody makes up a new locking mechanism, THEY 
> ALWAYS GET IT WRONG. Don't do it.
>
> I suggested using the rwsem locking for a good reason. It made sense. It 
> was simpler. Just do it that way, stop making up crap.

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?

Rafael
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