[Intel-gfx] [PATCH 4/5] drm/i915/execlists: Split spinlock from its irq disabling side-effect

[Chris Wilson]

Quoting Mika Kuoppala (2018-03-02 15:50:53)
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> 
> > During reset/wedging, we have to clean up the requests on the timeline
> > and flush the pending interrupt state. Currently, we are abusing the irq
> > disabling of the timeline spinlock to protect the irq state in
> > conjunction to the engine's timeline requests, but this is accidental
> > and conflates the spinlock with the irq state. A baffling state of
> > affairs for the reader.
> >
> > Instead, explicitly disable irqs over the critical section, and separate
> > modifying the irq state from the timeline's requests.
> >
> > Suggested-by: Mika Kuoppala <mika.kuoppala at linux.intel.com>
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
> > Cc: Michel Thierry <michel.thierry at intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_lrc.c | 21 +++++++++++++++------
> >  1 file changed, 15 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> > index 0482e54c94f0..7d1109aceabb 100644
> > --- a/drivers/gpu/drm/i915/intel_lrc.c
> > +++ b/drivers/gpu/drm/i915/intel_lrc.c
> > @@ -689,11 +689,13 @@ static void execlists_cancel_requests(struct intel_engine_cs *engine)
> >  
> >       GEM_TRACE("%s\n", engine->name);
> >  
> > -     spin_lock_irqsave(&engine->timeline->lock, flags);
> > +     local_irq_save(flags);
> 
> Chris explained in irc that this is for lockdep only. It was a bit
> confusing as we already are guaranteed exclusive access to
> state by tasklet being killed and dead at this point.
> 
> I think this warrants a comment that this is to soothe lockdep.

/*
 * Before we call engine->cancel_requests(), we should have exclusive
 * access to the submission state. This is arranged for us by the caller
 * disabling the interrupt generation, the tasklet and other threads
 * that may then access the same state, giving us a free hand to
 * reset state. However, we still need to let lockdep be aware that
 * we know this state may be accessed in hardirq context, so we
 * disable the irq around this manipulation and we want to keep
 * the spinlock focused on its duties and not accidentally conflate
 * coverage to the submission's irq state. (Similarly, although we
 * shouldn't need to disable irq around the manipulation of the
 * submission's irq state, we also wish to remind ourselves that
 * it is irq state.)
 */

> >  
> >       /* Cancel the requests on the HW and clear the ELSP tracker. */
> >       execlists_cancel_port_requests(execlists);
> >  
> > +     spin_lock(&engine->timeline->lock);
> > @@ -1618,10 +1622,11 @@ static void reset_common_ring(struct intel_engine_cs *engine,
> >       GEM_TRACE("%s seqno=%x\n",
> >                 engine->name, request ? request->global_seqno : 0);
> >  
> > -     spin_lock_irqsave(&engine->timeline->lock, flags);

	/* See execlists_cancel_requests() for the irq/spinlock split. */
> > +     local_irq_save(flags);

Good?
-Chris