Re: [PATCH 3/6] drm/gem: use new ww_mutex_(un)lock_for_each macros

[Christian KÃnig]

Am 14.06.19 um 14:59 schrieb Peter Zijlstra:
> On Fri, Jun 14, 2019 at 02:41:22PM +0200, Christian KÃnig wrote:
> > Use the provided macros instead of implementing deadlock handling on our own.
> >
> > Signed-off-by: Christian KÃnig <christian.koenig@xxxxxxx>
> > ---
> >   drivers/gpu/drm/drm_gem.c | 49 ++++++++++-----------------------------
> >   1 file changed, 12 insertions(+), 37 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> > index 50de138c89e0..6e4623d3bee2 100644
> > --- a/drivers/gpu/drm/drm_gem.c
> > +++ b/drivers/gpu/drm/drm_gem.c
> > @@ -1307,51 +1307,26 @@ int
> >   drm_gem_lock_reservations(struct drm_gem_object **objs, int count,
> >   			  struct ww_acquire_ctx *acquire_ctx)
> >   {
> > -	int contended = -1;
> > +	struct ww_mutex *contended;
> >   	int i, ret;
> >   
> >   	ww_acquire_init(acquire_ctx, &reservation_ww_class);
> >   
> > -retry:
> > -	if (contended != -1) {
> > -		struct drm_gem_object *obj = objs[contended];
> > -
> > -		ret = ww_mutex_lock_slow_interruptible(&obj->resv->lock,
> > -						       acquire_ctx);
> > -		if (ret) {
> > -			ww_acquire_done(acquire_ctx);
> > -			return ret;
> > -		}
> > -	}
> > -
> > -	for (i = 0; i < count; i++) {
> > -		if (i == contended)
> > -			continue;
> > -
> > -		ret = ww_mutex_lock_interruptible(&objs[i]->resv->lock,
> > -						  acquire_ctx);
> > -		if (ret) {
> > -			int j;
> > -
> > -			for (j = 0; j < i; j++)
> > -				ww_mutex_unlock(&objs[j]->resv->lock);
> > -
> > -			if (contended != -1 && contended >= i)
> > -				ww_mutex_unlock(&objs[contended]->resv->lock);
> > -
> > -			if (ret == -EDEADLK) {
> > -				contended = i;
> > -				goto retry;
> retry here, starts the whole locking loop.
>
> > -			}
> > -
> > -			ww_acquire_done(acquire_ctx);
> > -			return ret;
> > -		}
> > -	}
> +#define ww_mutex_unlock_for_each(loop, pos, contended) \
> +       if (!IS_ERR(contended)) {                       \
> +               if (contended)                          \
> +                       ww_mutex_unlock(contended);     \
> +               contended = (pos);                      \
> +               loop {                                  \
> +                       if (contended == (pos))         \
> +                               break;                  \
> +                       ww_mutex_unlock(pos);           \
> +               }                                       \
> +       }
> +
>
> +#define ww_mutex_lock_for_each(loop, pos, contended, ret, intr, ctx)   \
> +       for (contended = ERR_PTR(-ENOENT); ({                           \
> +               __label__ relock, next;                                 \
> +               ret = -ENOENT;                                          \
> +               if (contended == ERR_PTR(-ENOENT))                      \
> +                       contended = NULL;                               \
> +               else if (contended == ERR_PTR(-EDEADLK))                \
> +                       contended = (pos);                              \
> +               else                                                    \
> +                       goto next;                                      \
> +               loop {                                                  \
> +                       if (contended == (pos)) {                       \
> +                               contended = NULL;                       \
> +                               continue;                               \
> +                       }                                               \
> +relock:                                                                        \
> +                       ret = !(intr) ? ww_mutex_lock(pos, ctx) :       \
> +                               ww_mutex_lock_interruptible(pos, ctx);  \
> +                       if (ret == -EDEADLK) {                          \
> +                               ww_mutex_unlock_for_each(loop, pos,     \
> +                                                        contended);    \
> +                               contended = ERR_PTR(-EDEADLK);          \
> +                               goto relock;                            \
>
> while relock here continues where it left of and doesn't restart @loop.
> Or am I reading this monstrosity the wrong way?

contended = ERR_PTR(-EDEADLK) makes sure that the whole loop is 
restarted after retrying to acquire the lock.

See the "if" above "loop".

Christian.

> +                       }                                               \
> +                       break;                                          \
> +next:                                                                  \
> +                       continue;                                       \
> +               }                                                       \
> +       }), ret != -ENOENT;)
> +
>
> > +	ww_mutex_lock_for_each(for (i = 0; i < count; i++),
> > +			       &objs[i]->resv->lock, contended, ret, true,
> > +			       acquire_ctx)
> > +		if (ret)
> > +			goto error;
> >   
> >   	ww_acquire_done(acquire_ctx);
> >   
> >   	return 0;
> > +
> > +error:
> > +	ww_mutex_unlock_for_each(for (i = 0; i < count; i++),
> > +				 &objs[i]->resv->lock, contended);
> > +	ww_acquire_done(acquire_ctx);
> > +	return ret;
> >   }
> >   EXPORT_SYMBOL(drm_gem_lock_reservations);