Re: [PATCH] drm/i915: Only free the unpin_work if cancelled before being run

[Chris Wilson <chris@chris-wilson.co.uk>]

On Tue, 17 Apr 2012 12:33:46 +0200, Daniel Vetter <daniel@ffwll.ch> wrote:
> On Tue, Apr 17, 2012 at 10:53:24AM +0100, Chris Wilson wrote:
> > On Tue, 17 Apr 2012 10:29:38 +0100, Chris Wilson <chris@chris-wilson.co.uk> wrote:
> > > The unpin worker frees it work struct and so during intel_crtc_disable
> > > we should only also free the work struct if cancel_work_sync() reports
> > > that it successfully cancelled the work prior to it being executed and
> > > thus avoid the double free.
> > > 
> > > The impact is only for people unloading modules during a fullscreen game
> > > or movie playback, so extremely small.
> > 
> > Futher review (hunting for some sign of workqueue corruption, cf
> > https://bugs.freedesktop.org/show_bug.cgi?id=48798) says that if work is
> > non-NULL here it will not have been scheduled so cancel_work_sync() will
> > always return true.
> 
> Well, I've failed to notice this while reviewing the unpin_work life-cycle
> ...
> 
> > Which also means that we have no way of waiting upon the scheduled
> > unpin_work. :|
> 
> ... but have noticed that we lose any reference from intel_crtc to the
> unpin work once it's scheduled, and checked that we properly flush the
> work queue: See the ordering of irq disable, flush workqueue, then crtc
> destroy (in mode_config_cleanup).
> 
> We even have a flush_workqueu in i915_dma.c before tearing down gem, but
> that won't work too well now that unpin also frobs around with the fbc
> state (which is gone by now). But there's still the misleading comment
> that this syncs up with unpin work.
> 
> Care to clean this up a bit by
> - ditching the unnecessary flush_workqueu in i915_dma.c
Indeed looks superfluous. If we consider the unlikelihood the cleanup
code being well-tested, I'd prefer that we did do something like
drain_workqueue() as the first step in unload().

> - move the comment about syncing up with unpin_work to where we actually
>   sync up

I'm not finding another point where we explicitly sync with outstanding
unpin work. Probably due to the comment being in the wrong place...

> - and kill the superflous cancel_work?
Which also reminds me that the unpin_work holds onto a few references
that need to be released as well as the kfree. See above about these code
paths being relatively untrod.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre