Re: [PATCH 02/13] drm/i915: Stop the machine whilst capturing the GPU crash dump

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, Aug 05, 2016 at 10:05:53AM +0100, Chris Wilson wrote:
> The error state is purposefully racy as we expect it to be called at any
> time and so have avoided any locking whilst capturing the crash dump.
> However, with multi-engine GPUs and multiple CPUs, those races can
> manifest into OOPSes as we attempt to chase dangling pointers freed on
> other CPUs. Under discussion are lots of ways to slow down normal
> operation in order to protect the post-mortem error capture, but what it
> we take the opposite approach and freeze the machine whilst the error
> capture runs (note the GPU may still running, but as long as we don't
> process any of the results the driver's bookkeeping will be static).
> 
> Note that by of itself, this is not a complete fix. It also depends on
> the compiler barriers in list_add/list_del to prevent traversing the
> lists into the void.

The other important bit I think are NULL checks. I think the commit
message should mention that too.
> 
> v2: Avoid drm_clflush_pages() inside stop_machine() as it may use
> stop_machine() itself for its wbinvd fallback.

Droppingt he clflush from error capture seems like a pretty big
regression, at least at a glance. Why does this not result in piles of
corrupted error state captures?

I guess since we need to flush on incoherent platforms before gpu access
anyway this is mostly true (as long as userspace doesn't do something
silly), but I think it should be captured in a comment at the place of the
clflush.
-Daniel

> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>  drivers/gpu/drm/i915/Kconfig          |  1 +
>  drivers/gpu/drm/i915/i915_drv.h       |  2 ++
>  drivers/gpu/drm/i915/i915_gpu_error.c | 48 +++++++++++++++++++++--------------
>  3 files changed, 32 insertions(+), 19 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/Kconfig b/drivers/gpu/drm/i915/Kconfig
> index 7769e469118f..7badcee88ebf 100644
> --- a/drivers/gpu/drm/i915/Kconfig
> +++ b/drivers/gpu/drm/i915/Kconfig
> @@ -4,6 +4,7 @@ config DRM_I915
>  	depends on X86 && PCI
>  	select INTEL_GTT
>  	select INTERVAL_TREE
> +	select STOP_MACHINE
>  	# we need shmfs for the swappable backing store, and in particular
>  	# the shmem_readpage() which depends upon tmpfs
>  	select SHMEM
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 7054baa92061..9a7ffa49cd1f 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -481,6 +481,8 @@ struct drm_i915_error_state {
>  	struct kref ref;
>  	struct timeval time;
>  
> +	struct drm_i915_private *i915;
> +
>  	char error_msg[128];
>  	bool simulated;
>  	int iommu;
> diff --git a/drivers/gpu/drm/i915/i915_gpu_error.c b/drivers/gpu/drm/i915/i915_gpu_error.c
> index ced296983caa..b94a59733cf8 100644
> --- a/drivers/gpu/drm/i915/i915_gpu_error.c
> +++ b/drivers/gpu/drm/i915/i915_gpu_error.c
> @@ -28,6 +28,7 @@
>   */
>  
>  #include <generated/utsrelease.h>
> +#include <linux/stop_machine.h>
>  #include "i915_drv.h"
>  
>  static const char *engine_str(int engine)
> @@ -684,14 +685,12 @@ i915_error_object_create(struct drm_i915_private *dev_priv,
>  
>  	dst->page_count = num_pages;
>  	while (num_pages--) {
> -		unsigned long flags;
>  		void *d;
>  
>  		d = kmalloc(PAGE_SIZE, GFP_ATOMIC);
>  		if (d == NULL)
>  			goto unwind;
>  
> -		local_irq_save(flags);
>  		if (use_ggtt) {
>  			void __iomem *s;
>  
> @@ -710,15 +709,10 @@ i915_error_object_create(struct drm_i915_private *dev_priv,
>  
>  			page = i915_gem_object_get_page(src, i);
>  
> -			drm_clflush_pages(&page, 1);
> -
>  			s = kmap_atomic(page);
>  			memcpy(d, s, PAGE_SIZE);
>  			kunmap_atomic(s);
> -
> -			drm_clflush_pages(&page, 1);
>  		}
> -		local_irq_restore(flags);
>  
>  		dst->pages[i++] = d;
>  		reloc_offset += PAGE_SIZE;
> @@ -1371,6 +1365,32 @@ static void i915_capture_gen_state(struct drm_i915_private *dev_priv,
>  	error->suspend_count = dev_priv->suspend_count;
>  }
>  
> +static int capture(void *data)
> +{
> +	struct drm_i915_error_state *error = data;
> +
> +	/* Ensure that what we readback from memory matches what the GPU sees */
> +	wbinvd();
> +
> +	i915_capture_gen_state(error->i915, error);
> +	i915_capture_reg_state(error->i915, error);
> +	i915_gem_record_fences(error->i915, error);
> +	i915_gem_record_rings(error->i915, error);
> +
> +	i915_capture_active_buffers(error->i915, error);
> +	i915_capture_pinned_buffers(error->i915, error);
> +
> +	do_gettimeofday(&error->time);
> +
> +	error->overlay = intel_overlay_capture_error_state(error->i915);
> +	error->display = intel_display_capture_error_state(error->i915);
> +
> +	/* And make sure we don't leave trash in the CPU cache */
> +	wbinvd();
> +
> +	return 0;
> +}
> +
>  /**
>   * i915_capture_error_state - capture an error record for later analysis
>   * @dev: drm device
> @@ -1399,19 +1419,9 @@ void i915_capture_error_state(struct drm_i915_private *dev_priv,
>  	}
>  
>  	kref_init(&error->ref);
> +	error->i915 = dev_priv;
>  
> -	i915_capture_gen_state(dev_priv, error);
> -	i915_capture_reg_state(dev_priv, error);
> -	i915_gem_record_fences(dev_priv, error);
> -	i915_gem_record_rings(dev_priv, error);
> -
> -	i915_capture_active_buffers(dev_priv, error);
> -	i915_capture_pinned_buffers(dev_priv, error);
> -
> -	do_gettimeofday(&error->time);
> -
> -	error->overlay = intel_overlay_capture_error_state(dev_priv);
> -	error->display = intel_display_capture_error_state(dev_priv);
> +	stop_machine(capture, error, NULL);
>  
>  	i915_error_capture_msg(dev_priv, error, engine_mask, error_msg);
>  	DRM_INFO("%s\n", error->error_msg);
> -- 
> 2.8.1
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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