Re: [PATCH] drm/i915: Ditch the i915_gem_ww_ctx loop member

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On 8/16/21 3:25 PM, Matthew Auld wrote:
> On Mon, 16 Aug 2021 at 09:49, Thomas Hellström
> <thomas.hellstrom@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@linux.intel.com>
>> Signed-off-by: Thomas Hellström <thomas.hellstrom@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?

Yes, good catch. We should either just clear the -EDEADLK case, or not 
clear the error at all..

/Thomas