Re: [Intel-gfx] [PATCH] drm/i915/gem: Avoid synchronous binds deep within locks

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

Chris Wilson <chris@chris-wilson.co.uk> writes:

> On bxt, we require a VT'd w/a to serialise all GGTT updates with memory
> transfers, and use stop_machine() for this purpose. stop_machine() is a
> global serialisation barrier and so dangerous to use from within
> critical sections, as the stop_machine() will wait for all cpus to enter
> the stop_machine callback, and those cpus may be waiting for the
> critical section already held.
>
> Fixes: d7085b0faac8 ("drm/i915/gem: Poison stolen pages before use")
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>

Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>

> ---
>  drivers/gpu/drm/i915/gem/i915_gem_stolen.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_stolen.c b/drivers/gpu/drm/i915/gem/i915_gem_stolen.c
> index 4f923b8c43fb..29bffc6afcc1 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_stolen.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_stolen.c
> @@ -505,6 +505,9 @@ static void dbg_poison(struct i915_ggtt *ggtt,
>  	if (!drm_mm_node_allocated(&ggtt->error_capture))
>  		return;
>  
> +	if (ggtt->vm.bind_async_flags & I915_VMA_GLOBAL_BIND)
> +		return; /* beware stop_machine() inversion */
> +
>  	GEM_BUG_ON(!IS_ALIGNED(size, PAGE_SIZE));
>  
>  	mutex_lock(&ggtt->error_mutex);
> -- 
> 2.20.1
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