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

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

On 8/16/21 3:34 PM, Maarten Lankhorst wrote:
> 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.

OK, I'll skip the clearing then.

/Thomas


>
> ~Maarten
>