[Intel-gfx] [PATCH] drm/i915: Ditch the i915_gem_ww_ctx loop member

[Matthew Auld]

On Mon, 16 Aug 2021 at 09:49, Thomas Hellström
<thomas.hellstrom at linux.intel.com> wrote:
>
> It's only used by the for_i915_gem_ww() macro and we can use
> the (typically) on-stack _err variable in its place.
>
> While initially setting the _err variable to -EDEADLK to enter the
> loop, we clear it before actually entering using fetch_and_zero() to
> avoid empty loops or code not setting the _err variable running forever.
>
> Suggested-by: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> Signed-off-by: Thomas Hellström <thomas.hellstrom at linux.intel.com>
> ---
>  drivers/gpu/drm/i915/i915_gem_ww.h | 23 ++++++++---------------
>  1 file changed, 8 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem_ww.h b/drivers/gpu/drm/i915/i915_gem_ww.h
> index f6b1a796667b..98348b1e6182 100644
> --- a/drivers/gpu/drm/i915/i915_gem_ww.h
> +++ b/drivers/gpu/drm/i915/i915_gem_ww.h
> @@ -7,12 +7,13 @@
>
>  #include <drm/drm_drv.h>
>
> +#include "i915_utils.h"
> +
>  struct i915_gem_ww_ctx {
>         struct ww_acquire_ctx ctx;
>         struct list_head obj_list;
>         struct drm_i915_gem_object *contended;
> -       unsigned short intr;
> -       unsigned short loop;
> +       bool intr;
>  };
>
>  void i915_gem_ww_ctx_init(struct i915_gem_ww_ctx *ctx, bool intr);
> @@ -23,28 +24,20 @@ void i915_gem_ww_unlock_single(struct drm_i915_gem_object *obj);
>  /* Internal functions used by the inlines! Don't use. */
>  static inline int __i915_gem_ww_fini(struct i915_gem_ww_ctx *ww, int err)
>  {
> -       ww->loop = 0;
>         if (err == -EDEADLK) {
>                 err = i915_gem_ww_ctx_backoff(ww);
>                 if (!err)
> -                       ww->loop = 1;
> +                       err = -EDEADLK;
>         }
>
> -       if (!ww->loop)
> +       if (err != -EDEADLK)
>                 i915_gem_ww_ctx_fini(ww);
>
>         return err;
>  }
>
> -static inline void
> -__i915_gem_ww_init(struct i915_gem_ww_ctx *ww, bool intr)
> -{
> -       i915_gem_ww_ctx_init(ww, intr);
> -       ww->loop = 1;
> -}
> -
> -#define for_i915_gem_ww(_ww, _err, _intr)                      \
> -       for (__i915_gem_ww_init(_ww, _intr); (_ww)->loop;       \
> +#define for_i915_gem_ww(_ww, _err, _intr)                        \
> +       for (i915_gem_ww_ctx_init(_ww, _intr), (_err) = -EDEADLK; \
> +            fetch_and_zero(&_err) == -EDEADLK;                   \

Doesn't this now hide "normal" errors, like say get_pages() returning
-ENOSPC or so?

>              _err = __i915_gem_ww_fini(_ww, _err))
> -
>  #endif
> --
> 2.31.1
>