Re: [PATCH 01/33] drm/i915: Add smp_rmb() to busy ioctl's RCU dance

[Daniel Vetter <daniel@ffwll.ch>]

On Sun, Aug 07, 2016 at 03:45:09PM +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 <chris@chris-wilson.co.uk>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

r-b confirmed.
-Daniel

> ---
>  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();
>  		if (request == rcu_access_pointer(active->request))
>  			return flag(id);
>  	} while (1);
>  }
>  
> -static inline unsigned
> +static __always_inline unsigned int
>  busy_check_reader(const struct i915_gem_active *active)
>  {
>  	return __busy_set_if_active(active, __busy_read_flag);
>  }
>  
> -static inline unsigned
> +static __always_inline unsigned int
>  busy_check_writer(const struct i915_gem_active *active)
>  {
>  	return __busy_set_if_active(active, __busy_write_id);
> diff --git a/drivers/gpu/drm/i915/i915_gem_request.h b/drivers/gpu/drm/i915/i915_gem_request.h
> index 3496e28785e7..b2456dede3ad 100644
> --- a/drivers/gpu/drm/i915/i915_gem_request.h
> +++ b/drivers/gpu/drm/i915/i915_gem_request.h
> @@ -497,6 +497,9 @@ __i915_gem_active_get_rcu(const struct i915_gem_active *active)
>  		 * incremented) then the following read for rcu_access_pointer()
>  		 * must occur after the atomic operation and so confirm
>  		 * that this request is the one currently being tracked.
> +		 *
> +		 * The corresponding write barrier is part of
> +		 * rcu_assign_pointer().
>  		 */
>  		if (!request || request == rcu_access_pointer(active->request))
>  			return rcu_pointer_handoff(request);
> -- 
> 2.8.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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