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

[Maarten Lankhorst <maarten.lankhorst@linux.intel.com>]

Op 16-08-2021 om 15:30 schreef Thomas Hellström:
>
> 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

I believe not setting _err is a bug anyway. Why would you do such a loop without at least one err = ww_mutex_lock(&ww); ?

Infinite loop would catch that at first test.

~Maarten