From 188e936b0e3c821ef9ec0806907d071a074eed89 Mon Sep 17 00:00:00 2001 From: Matthew Auld Date: Tue, 21 Sep 2021 14:42:02 +0100 Subject: [PATCH] drm/i915/request: fix early tracepoints Currently we blow up in trace_dma_fence_init, when calling into get_driver_name or get_timeline_name, since both the engine and context might be NULL(or contain some garbage address) in the case of newly allocated slab objects via the request ctor. Note that we also use SLAB_TYPESAFE_BY_RCU here, which allows requests to be immediately freed, but delay freeing the underlying page by an RCU grace period. With this scheme requests can be re-allocated, at the same time as they are also being read by some lockless RCU lookup mechanism. In the ctor case, which is only called for new slab objects(i.e allocate new page and call the ctor for each object) it's safe to reset the context/engine prior to calling into dma_fence_init, since we can be certain that no one is doing an RCU lookup which might depend on peeking at the engine/context, like in active_engine(), since the object can't yet be externally visible. In the recycled case(which might also be externally visible) the request refcount always transitions from 0->1 after we set the context/engine etc, which should ensure it's valid to dereference the engine for example, when doing an RCU list-walk, so long as we can also increment the refcount first. If the refcount is already zero, then the request is considered complete/released. If it's non-zero, then the request might be in the process of being re-allocated, or potentially still in flight, however after successfully incrementing the refcount, it's possible to carefully inspect the request state, to determine if the request is still what we were looking for. Note that all externally visible requests returned to the cache must have zero refcount. One possible fix then is to move dma_fence_init out from the request ctor. Originally this was how it was done, but it was moved in: commit e9f02341ca86d7b08245600f54a1946bc82bbbf4 Author: Chris Wilson Date: Mon Feb 3 09:41:48 2020 +0000 drm/i915: Initialise basic fence before acquiring seqno where it looks like intel_timeline_get_seqno() relied on some of the rq->fence state, but that is no longer the case since: commit 19dfa3d3ed3b2b2e810da2a78b1329c295c0e96a Author: Maarten Lankhorst Date: Tue Mar 23 16:49:50 2021 +0100 drm/i915: Do not share hwsp across contexts any more, v8. intel_timeline_get_seqno() could also be cleaned up slightly by dropping the request argument. Moving dma_fence_init back out of the ctor, should ensure we have enough of the request initialised in case of trace_dma_fence_init. Functionally this should be the same, and is effectively what we were already open coding before, except now we also assign the fence->lock and fence->ops, but since these are invariant for recycled requests(which might be externally visible), and will therefore already hold the same value, it shouldn't matter. An alternative fix, since we don't yet have a fully initialised request when in the ctor, is just setting the context/engine as NULL, but this does require adding some extra handling in get_driver_name etc. v2(Daniel): - Try to make the commit message less confusing Fixes: e9f02341ca86 ("drm/i915: Initialise basic fence before acquiring seqno") Signed-off-by: Matthew Auld Cc: Michael Mason Cc: Daniel Vetter Reviewed-by: Daniel Vetter Link: https://patchwork.freedesktop.org/patch/msgid/20210921134202.3803151-1-matthew.auld@intel.com --- drivers/gpu/drm/i915/i915_request.c | 11 ++--------- 1 file changed, 2 insertions(+), 9 deletions(-) diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c index ce446716d0929..79da5eca60af5 100644 --- a/drivers/gpu/drm/i915/i915_request.c +++ b/drivers/gpu/drm/i915/i915_request.c @@ -829,8 +829,6 @@ static void __i915_request_ctor(void *arg) i915_sw_fence_init(&rq->submit, submit_notify); i915_sw_fence_init(&rq->semaphore, semaphore_notify); - dma_fence_init(&rq->fence, &i915_fence_ops, &rq->lock, 0, 0); - rq->capture_list = NULL; init_llist_head(&rq->execute_cb); @@ -905,17 +903,12 @@ __i915_request_create(struct intel_context *ce, gfp_t gfp) rq->ring = ce->ring; rq->execution_mask = ce->engine->mask; - kref_init(&rq->fence.refcount); - rq->fence.flags = 0; - rq->fence.error = 0; - INIT_LIST_HEAD(&rq->fence.cb_list); - ret = intel_timeline_get_seqno(tl, rq, &seqno); if (ret) goto err_free; - rq->fence.context = tl->fence_context; - rq->fence.seqno = seqno; + dma_fence_init(&rq->fence, &i915_fence_ops, &rq->lock, + tl->fence_context, seqno); RCU_INIT_POINTER(rq->timeline, tl); rq->hwsp_seqno = tl->hwsp_seqno; -- 2.39.5