Re: [Intel-gfx] [PATCH 2/2] drm/i915: Avoid using rq->engine after free during i915_fence_release

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

Quoting Chris Wilson (2020-05-21 10:32:49)
> Quoting Chris Wilson (2020-05-21 10:27:16)
> > Quoting Tvrtko Ursulin (2020-05-21 10:13:14)
> > > 
> > > On 21/05/2020 09:53, Chris Wilson wrote:
> > > > In order to be valid to dereference during the i915_fence_release, after
> > > > retiring the fence and releasing its refererences, we assume that
> > > > rq->engine can only be a real engine (that stay intact until the device
> > > > is shutdown after all fences have been flushed). However, due to a quirk
> > > > of preempt-to-busy, we may retire a request that still belongs to a
> > > > virtual engine and so eventually free it with rq->engine being invalid.
> > > > To avoid dereferencing that invalid engine, we look at the
> > > > execution_mask which if it indicates it may be executed on more than one
> > > > engine, we know it originated on a virtual engine and may still be on
> > > > one.
> > > > 
> > > > Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/1906
> > > > Fixes: 43acd6516ca9 ("drm/i915: Keep a per-engine request pool")
> > > > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > > > Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> > > > ---
> > > >   drivers/gpu/drm/i915/i915_request.c | 25 +++++++++++++++++++++++--
> > > >   1 file changed, 23 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> > > > index 526c1e9acbd5..6e357183bece 100644
> > > > --- a/drivers/gpu/drm/i915/i915_request.c
> > > > +++ b/drivers/gpu/drm/i915/i915_request.c
> > > > @@ -121,8 +121,29 @@ static void i915_fence_release(struct dma_fence *fence)
> > > >       i915_sw_fence_fini(&rq->submit);
> > > >       i915_sw_fence_fini(&rq->semaphore);
> > > >   
> > > > -     /* Keep one request on each engine for reserved use under mempressure */
> > > > -     if (!cmpxchg(&rq->engine->request_pool, NULL, rq))
> > > > +     /*
> > > > +      * Keep one request on each engine for reserved use under mempressure
> > > > +      *
> > > > +      * We do not hold a reference to the engine here and so have to be
> > > > +      * very careful in what rq->engine we poke. The virtual engine is
> > > > +      * referenced via the rq->context and we released that ref during
> > > > +      * i915_request_retire(), ergo we must not dereference a virtual
> > > > +      * engine here. Not that we would want to, as the only consumer of
> > > > +      * the reserved engine->request_pool is the powermanagent parking,
> > > 
> > > power management
> > > 
> > > > +      * which must-not-fail, and that is only run on the physical engines.
> > > > +      *
> > > > +      * Since the request must have been executed to be have completed,
> > > > +      * we know that it will have been processed by the HW and will
> > > > +      * not be unsubmitted again, so rq->engine and rq->execution_mask
> > > > +      * at this point is stable. rq->execution_mask will be a single
> > > > +      * bit if the last and only engine it could execution on was a
> > > > +      * physical engine, if it's multiple bits then it started on and
> > > > +      * could still be on a virtual engine. Thus if the mask is not a
> > > > +      * power-of-two we assume that rq->engine may still be a virtual
> > > > +      * engien and so a dangling invalid pointer that we cannot 
> > > 
> > > engine
> > > 
> > > But.. submit fence can mask out execution_mask bits and make it appear 
> > > the request was on a physical engine. What then?
> > 
> > Then we execute along a single engine and it is never returned to the
> > virtual engine (in __unwind_incomplete_requests). 
> 
>          * For example, consider the flow of a bonded request through a virtual
>          * engine. The request is created with a wide engine mask (all engines
>          * that we might execute on). On processing the bond, the request mask
>          * is reduced to one or more engines. If the request is subsequently
>          * bound to a single engine, it will then be constrained to only
>          * execute on that engine and never returned to the virtual engine
>          * after timeslicing away, see __unwind_incomplete_requests(). Thus we
>          * know that if the rq->execution_mask is a single bit, only rq->engine

rq->engine can only be a physical engine, with the exact corresponding mask.

>          * can be the exact corresponding engine->mask.
> 
> -Chris
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