This reverts commit bcb9aa45d5a0e11ef91245330c53cde214d15e8d. The problem with this patch is that it makes rq->engine dangling pointer after context removal. At least two places were located where it ends with errors: - i915_fence_release: intel_engine_is_virtual(rq->engine), - i915_request_wait_timeout: mutex_(acquire|release)(&rq->engine->gt...) It is possible there are more such places. Taking context reference prevents this behaviour.
Commit message suggested the patch prevented reference loop in VM_BIND, but since VM_BIND was dropped the revert hopefuly should be safe. The patch fixes bug detected by KASAN with following signature: i915 0000:00:02.0: [drm:i915_drop_caches_set [i915]] Dropping caches: 0x0000005c [0x0000005c] BUG: KASAN: slab-use-after-free in i915_fence_release+0x2a2/0x2f0 [i915] Read of size 4 at addr ffff88813ffda6e8 by task kworker/u32:10/157 ... Allocated by task 1184: ... guc_create_virtual+0x4d/0x1160 [i915] i915_gem_create_context+0x11ee/0x18c0 [i915] ... Freed by task 151: ... intel_guc_deregister_done_process_msg+0x324/0x6d0 [i915] ... Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/7926 Signed-off-by: Andrzej Hajda <andrzej.ha...@intel.com> --- Hi Niranjana, Matt, Could you look at this revert, to verify if situation described in reverted patch will not happen now, without VM_BIND. Regards Andrzej --- drivers/gpu/drm/i915/i915_request.c | 52 +++++++++++------------------ 1 file changed, 20 insertions(+), 32 deletions(-) diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c index 894068bb37b6f1..c5fe199b046d01 100644 --- a/drivers/gpu/drm/i915/i915_request.c +++ b/drivers/gpu/drm/i915/i915_request.c @@ -134,42 +134,17 @@ static void i915_fence_release(struct dma_fence *fence) i915_sw_fence_fini(&rq->semaphore); /* - * Keep one request on each engine for reserved use under mempressure + * Keep one request on each engine for reserved use under mempressure, * do not use with virtual engines as this really is only needed for * kernel contexts. - * - * 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 (!intel_engine_is_virtual(rq->engine) && - is_power_of_2(rq->execution_mask) && - !cmpxchg(&rq->engine->request_pool, NULL, rq)) + !cmpxchg(&rq->engine->request_pool, NULL, rq)) { + intel_context_put(rq->context); return; + } + + intel_context_put(rq->context); kmem_cache_free(slab_requests, rq); } @@ -946,7 +921,19 @@ __i915_request_create(struct intel_context *ce, gfp_t gfp) } } - rq->context = ce; + /* + * Hold a reference to the intel_context over life of an i915_request. + * Without this an i915_request can exist after the context has been + * destroyed (e.g. request retired, context closed, but user space holds + * a reference to the request from an out fence). In the case of GuC + * submission + virtual engine, the engine that the request references + * is also destroyed which can trigger bad pointer dref in fence ops + * (e.g. i915_fence_get_driver_name). We could likely change these + * functions to avoid touching the engine but let's just be safe and + * hold the intel_context reference. In execlist mode the request always + * eventually points to a physical engine so this isn't an issue. + */ + rq->context = intel_context_get(ce); rq->engine = ce->engine; rq->ring = ce->ring; rq->execution_mask = ce->engine->mask; @@ -1022,6 +1009,7 @@ __i915_request_create(struct intel_context *ce, gfp_t gfp) GEM_BUG_ON(!list_empty(&rq->sched.waiters_list)); err_free: + intel_context_put(ce); kmem_cache_free(slab_requests, rq); err_unreserve: intel_context_unpin(ce); -- 2.34.1
