Re: [PATCH v2] drm/i915: Move the mb() following release-mmap into release-mmap

From: Daniel Vetter <daniel@ffwll.ch>
To: Chris Wilson <chris@chris-wilson.co.uk>
Cc: intel-gfx@lists.freedesktop.org, "Goel, Akash" <akash.goel@intel.com>
Subject: Re: [PATCH v2] drm/i915: Move the mb() following release-mmap into release-mmap
Date: Tue, 24 Nov 2015 14:36:43 +0100	[thread overview]
Message-ID: <20151124133643.GA17050@phenom.ffwll.local> (raw)
In-Reply-To: <1448015498-24846-1-git-send-email-chris@chris-wilson.co.uk>

On Fri, Nov 20, 2015 at 10:31:38AM +0000, Chris Wilson wrote:
> As paranoia, we want to ensure that the CPU's PTEs have been revoked for
> the object before we return from i915_gem_release_mmap(). This allows us
> to rely on there being no outstanding memory accesses and guarantees
> serialisation of the code against concurrent access just by calling
> i915_gem_release_mmap().
> 
> v2: Reduce the mb() into a wmb() following the revoke.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
> Cc: "Goel, Akash" <akash.goel@intel.com
> ---
>  drivers/gpu/drm/i915/i915_gem.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index df234d00b376..09c829f38786 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -1903,11 +1903,21 @@ out:
>  void
>  i915_gem_release_mmap(struct drm_i915_gem_object *obj)
>  {
> +	/* Serialisation between user GTT access and our code depends upon
> +	 * revoking the CPU's PTE whilst the mutex is held. The next user
> +	 * pagefault then has to wait until we release the mutex.
> +	 */
> +	lockdep_assert_held(&obj->base.dev->struct_mutex);

lockdep_assert_held is a nop without lockdep, that's why I prefer
WARN_ON(!mutex_is_locked). Either way:

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

> +
>  	if (!obj->fault_mappable)
>  		return;
>  
>  	drm_vma_node_unmap(&obj->base.vma_node,
>  			   obj->base.dev->anon_inode->i_mapping);
> +
> +	/* Ensure that the CPU's PTE are revoked before we return */
> +	wmb();
> +
>  	obj->fault_mappable = false;
>  }
>  
> @@ -3212,9 +3222,6 @@ static void i915_gem_object_finish_gtt(struct drm_i915_gem_object *obj)
>  	if ((obj->base.read_domains & I915_GEM_DOMAIN_GTT) == 0)
>  		return;
>  
> -	/* Wait for any direct GTT access to complete */
> -	mb();
> -
>  	old_read_domains = obj->base.read_domains;
>  	old_write_domain = obj->base.write_domain;
>  
> -- 
> 2.6.2
> 
> _______________________________________________
> Intel-gfx mailing list
> Intel-gfx@lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/intel-gfx

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