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 <ch...@chris-wilson.co.uk> Cc: Tvrtko Ursulin <tvrtko.ursu...@intel.com> Reviewed-by: Tvrtko Ursulin <tvrtko.ursu...@intel.com> --- drivers/gpu/drm/i915/i915_request.c | 35 +++++++++++++++++++++++++++-- 1 file changed, 33 insertions(+), 2 deletions(-) diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c index 526c1e9acbd5..c282719ad3ac 100644 --- a/drivers/gpu/drm/i915/i915_request.c +++ b/drivers/gpu/drm/i915/i915_request.c @@ -121,8 +121,39 @@ 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 power management parking, + * 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 + * engine and so a dangling invalid pointer that we cannot dereference + * + * 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, rq->engine + * can be a physical engine with the exact corresponding mask. + */ + if (is_power_of_2(rq->execution_mask) && + !cmpxchg(&rq->engine->request_pool, NULL, rq)) return; kmem_cache_free(global.slab_requests, rq); -- 2.20.1 _______________________________________________ Intel-gfx mailing list Intel-gfx@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/intel-gfx
