Re: [PATCH 2/2] drm/i915: Protect engine request list with spinlock

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

Chris Wilson <chris@chris-wilson.co.uk> writes:

> 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
>

What I think is the failure mode on [1] is:

Request gets added to the ring but not yet
into the ring->request_list, gpu finishes it and updates
the hw status page. Hangcheck runs and sees that request_list
does not contain the supposed request and complains
that the hangcheck was activated on idle ring.

https://bugs.freedesktop.org/show_bug.cgi?id=88651

-Mika

> -- 
> Chris Wilson, Intel Open Source Technology Centre
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