[Intel-gfx] [PATCH 2/2] drm/i915: Protect engine request list with spinlock

[Chris Wilson]

On Tue, Feb 24, 2015 at 11:39:08AM +0100, Daniel Vetter wrote:
> On Tue, Feb 24, 2015 at 08:31:18AM +0000, Chris Wilson wrote:
> > On Tue, Feb 24, 2015 at 12:58:19AM +0100, Daniel Vetter wrote:
> > > On Thu, Feb 19, 2015 at 04:41:12PM +0000, Chris Wilson wrote:
> > > > On Thu, Feb 19, 2015 at 06:18:55PM +0200, Mika Kuoppala wrote:
> > > > > There are multiple players interested in the ring->request_list
> > > > > state. Request submission can happen in kernel or user context,
> > > > > idle worker is going through request list to free items. And then there
> > > > > is hangcheck worker which tries to figure out if particular ring is
> > > > > healthy by peeking at the request list among other things. And if
> > > > > judged stuck by hangcheck, error state is colleted. Which in turns
> > > > > needs access to ring->request_list.
> > > > 
> > > > We have discussed this before. Hangcheck does not need the lock so long
> > > > as it is serialised with deletion. List processing with hangcheck during
> > > > concurrent addition is safe.
> > > > 
> > > > For example, I expect the request locking to look like
> > > > 
> > > > http://cgit.freedesktop.org/~ickle/linux-2.6/tree/drivers/gpu/drm/i915/i915_gem_request.c#n691
> > > 
> > > I think longer-term with per-engine reset and fun stuff like that we
> > > probably want the spinlock, just to avoid too many headaches with locking
> > > auditing. For the execbuf fastpath it should just be one more spinlock per
> > > ioctl, so hopefully bearable.
> > 
> > But it is not even the locking bug that breaks capture, so what's the
> > point?
> 
> Oh I've read the patch as general prep work for more finegrained reset
> support not as a fix for the referenced bug. I guess the bug is just the
> usual incoherent seqno/irq thing that's been plagueing us ever since gen6?

I presumed Mika wants to fix that hangcheck and capture may explode as
requests are completed concurrently. The bug that I expect will remain
is that we peek at the bo without locks during capture.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre