Re: [PATCH] drm/i915: Avoid context dereference inside execlists_submission_tasklet

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

Quoting Michel Thierry (2017-12-19 22:43:57)
> On 12/19/2017 2:09 PM, Chris Wilson wrote:
> > A lesson that has to be relearnt over and over again is that the request
> > does not keep a reference to the context and so we cannot freely
> > dereference the context from inside the execlists_submission_tasklet. In
> > particular, we try to do so in the new GEM_TRACE() so convert those over
> > to the port->context_id we keep for GEM debugging. This means the
> > tracing now depends on DRM_I915_GEM_DEBUG.
> > 
> 
> Even before the port->context_id dependency, I don't think many people 
> would enable DRM_I915_TRACE_GEM without DRM_I915_DEBUG_GEM.

Indeed :)

> > Fixes: bccd3b831185 ("drm/i915: Use trace_printk to provide a death rattle for GEM")
> > References: https://bugs.freedesktop.org/show_bug.cgi?id=104066
> > References: https://bugs.freedesktop.org/show_bug.cgi?id=104162
> > References: https://bugs.freedesktop.org/show_bug.cgi?id=104242
> > References: https://bugs.freedesktop.org/show_bug.cgi?id=104310
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
> > Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> > ---
> >   drivers/gpu/drm/i915/Kconfig.debug | 2 +-
> >   drivers/gpu/drm/i915/intel_lrc.c   | 6 +++---
> >   2 files changed, 4 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/Kconfig.debug b/drivers/gpu/drm/i915/Kconfig.debug
> > index fa36491495b1..c846b250b9c4 100644
> > --- a/drivers/gpu/drm/i915/Kconfig.debug
> > +++ b/drivers/gpu/drm/i915/Kconfig.debug
> > @@ -29,7 +29,6 @@ config DRM_I915_DEBUG
> >          select SW_SYNC # signaling validation framework (igt/syncobj*)
> >          select DRM_I915_SW_FENCE_DEBUG_OBJECTS
> >          select DRM_I915_SELFTEST
> > -       select DRM_I915_TRACE_GEM
> >           default n
> >           help
> >             Choose this option to turn on extra driver debugging that may affect
> > @@ -55,6 +54,7 @@ config DRM_I915_TRACE_GEM
> >          bool "Insert extra ftrace output from the GEM internals"
> >          select TRACING
> >          default n
> > +       depends on DRM_I915_DEBUG_GEM
> >          help
> >            Enable additional and verbose debugging output that will spam
> >            ordinary tests, but may be vital for post-mortem debugging when
> > diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> > index eee718e3f371..64d49d5054b9 100644
> > --- a/drivers/gpu/drm/i915/intel_lrc.c
> > +++ b/drivers/gpu/drm/i915/intel_lrc.c
> > @@ -449,7 +449,7 @@ static void execlists_submit_ports(struct intel_engine_cs *engine)
> > 
> >                          GEM_TRACE("%s in[%d]:  ctx=%d.%d, seqno=%x\n",
> >                                    engine->name, n,
> > -                                 rq->ctx->hw_id, count,
> > +                                 port[n].context_id, count,
> >                                    rq->global_seqno);
> >                  } else {
> >                          GEM_BUG_ON(!n);
> > @@ -861,7 +861,7 @@ static void execlists_submission_tasklet(unsigned long data)
> >                           */
> > 
> >                          status = READ_ONCE(buf[2 * head]); /* maybe mmio! */
> > -                       GEM_TRACE("%s csb[%dd]: status=0x%08x:0x%08x\n",
> > +                       GEM_TRACE("%s csb[%d]: status=0x%08x:0x%08x\n",
> >                                    engine->name, head,
> >                                    status, buf[2*head + 1]);
> > 
> > @@ -905,7 +905,7 @@ static void execlists_submission_tasklet(unsigned long data)
> >                          rq = port_unpack(port, &count);
> >                          GEM_TRACE("%s out[0]: ctx=%d.%d, seqno=%x\n",
> >                                    engine->name,
> > -                                 rq->ctx->hw_id, count,
> > +                                 port->context_id, count,
> >                                    rq->global_seqno);
> >                          GEM_BUG_ON(count == 0);
> >                          if (--count == 0) {
> > --
> > 2.15.1
> > 
> 
> Reviewed-by: Michel Thierry <michel.thierry@intel.com>

Thanks, I'm looking forward to seeing how many incompletes now
evaporate.
-Chris
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