On Fri, Aug 05, 2016 at 10:13:22PM +0100, Chris Wilson wrote:
> In the debate as to whether the second read of active->request is
> ordered after the dependent reads of the first read of active->request,
> just give in and throw a smp_rmb() in there so that ordering of loads is
> assured.
>
> v2: Explain the manual smp_rmb()
>
> Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
> Cc: Daniel Vetter <daniel.vet...@ffwll.ch>
> Reviewed-by: Daniel Vetter <daniel.vet...@ffwll.ch>
> ---
> drivers/gpu/drm/i915/i915_gem.c | 25 ++++++++++++++++++++-----
> drivers/gpu/drm/i915/i915_gem_request.h | 3 +++
> 2 files changed, 23 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index f4f8eaa90f2a..654f0b015f97 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -3735,7 +3735,7 @@ i915_gem_object_ggtt_unpin_view(struct
> drm_i915_gem_object *obj,
> i915_vma_unpin(i915_gem_obj_to_ggtt_view(obj, view));
> }
>
> -static __always_inline unsigned __busy_read_flag(unsigned int id)
> +static __always_inline unsigned int __busy_read_flag(unsigned int id)
> {
> /* Note that we could alias engines in the execbuf API, but
> * that would be very unwise as it prevents userspace from
> @@ -3753,7 +3753,7 @@ static __always_inline unsigned int
> __busy_write_id(unsigned int id)
> return id;
> }
>
> -static __always_inline unsigned
> +static __always_inline unsigned int
> __busy_set_if_active(const struct i915_gem_active *active,
> unsigned int (*flag)(unsigned int id))
> {
> @@ -3770,19 +3770,34 @@ __busy_set_if_active(const struct i915_gem_active
> *active,
>
> id = request->engine->exec_id;
>
> - /* Check that the pointer wasn't reassigned and overwritten. */
> + /* Check that the pointer wasn't reassigned and overwritten.
> + *
> + * In __i915_gem_active_get_rcu(), we enforce ordering between
> + * the first rcu pointer dereference (imposing a
> + * read-dependency only on access through the pointer) and
> + * the second lockless access through the memory barrier
> + * following a successful atomic_inc_not_zero(). Here there
> + * is no such barrier, and so we must manually insert an
> + * explicit read barrier to ensure that the following
> + * access occurs after all the loads through the first
> + * pointer.
> + *
> + * The corresponding write barrier is part of
> + * rcu_assign_pointer().
> + */
> + smp_rmb();
Are you sure this should not just be a read_barrier_depends()?
active->request is data dependent on the earlier reads through it, and
here we are only caring that those loads are completed before we double
check the request hasn't been overwritten.
-Chris
--
Chris Wilson, Intel Open Source Technology Centre
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