From 70cb472c6d7a3575a9b3fc7e0188384b7ca3d705 Mon Sep 17 00:00:00 2001 From: Chris Wilson Date: Tue, 9 Aug 2016 18:08:25 +0100 Subject: [PATCH] drm/i915: Always mark the writer as also a read for busy ioctl One of the few guarantees we want the busy ioctl to provide is that the reported busy writer is included in the set of busy read engines. This should be provided by the ordering of setting and retiring the active trackers, but we can do better by explicitly setting the busy read engine flag for the last writer. v2: More comments inside __busy_write_id() to explain why both fields are set. Fixes: 3fdc13c7a3cb ("drm/i915: Remove (struct_mutex) locking for busy-ioctl") Signed-off-by: Chris Wilson Cc: Daniel Vetter Cc: Joonas Lahtinen Link: http://patchwork.freedesktop.org/patch/msgid/1470762505-12799-1-git-send-email-chris@chris-wilson.co.uk Reviewed-by: Daniel Vetter --- drivers/gpu/drm/i915/i915_gem.c | 18 ++++++++++++++---- 1 file changed, 14 insertions(+), 4 deletions(-) diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c index 373136f..3fb5749 100644 --- a/drivers/gpu/drm/i915/i915_gem.c +++ b/drivers/gpu/drm/i915/i915_gem.c @@ -3748,7 +3748,15 @@ static __always_inline unsigned int __busy_read_flag(unsigned int id) static __always_inline unsigned int __busy_write_id(unsigned int id) { - return id; + /* The uABI guarantees an active writer is also amongst the read + * engines. This would be true if we accessed the activity tracking + * under the lock, but as we perform the lookup of the object and + * its activity locklessly we can not guarantee that the last_write + * being active implies that we have set the same engine flag from + * last_read - hence we always set both read and write busy for + * last_write. + */ + return id | __busy_read_flag(id); } static __always_inline unsigned int @@ -3857,9 +3865,11 @@ i915_gem_busy_ioctl(struct drm_device *dev, void *data, args->busy |= busy_check_reader(&obj->last_read[idx]); /* For ABI sanity, we only care that the write engine is in - * the set of read engines. This is ensured by the ordering - * of setting last_read/last_write in i915_vma_move_to_active, - * and then in reverse in retire. + * the set of read engines. This should be ensured by the + * ordering of setting last_read/last_write in + * i915_vma_move_to_active(), and then in reverse in retire. + * However, for good measure, we always report the last_write + * request as a busy read as well as being a busy write. * * We don't care that the set of active read/write engines * may change during construction of the result, as it is -- 2.7.4