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Date:   Fri, 14 Jun 2019 14:59:40 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Christian =?iso-8859-1?Q?K=F6nig?= 
        <ckoenig.leichtzumerken@gmail.com>
Cc:     daniel@ffwll.ch, l.stach@pengutronix.de,
        linux+etnaviv@armlinux.org.uk, christian.gmeiner@gmail.com,
        yuq825@gmail.com, eric@anholt.net, thellstrom@vmware.com,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        etnaviv@lists.freedesktop.org, lima@lists.freedesktop.org
Subject: Re: [PATCH 3/6] drm/gem: use new ww_mutex_(un)lock_for_each macros
Message-ID: <20190614125940.GP3436@hirez.programming.kicks-ass.net>
References: <20190614124125.124181-1-christian.koenig@amd.com>
 <20190614124125.124181-4-christian.koenig@amd.com>
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On Fri, Jun 14, 2019 at 02:41:22PM +0200, Christian K?nig wrote:
> Use the provided macros instead of implementing deadlock handling on our own.
> 
> Signed-off-by: Christian K?nig <christian.koenig@amd.com>
> ---
>  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?

+                       }                                               \
+                       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);