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Date:   Fri, 14 Jun 2019 15:06:10 +0200
From:   Christian König <ckoenig.leichtzumerken@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     daniel@...ll.ch, l.stach@...gutronix.de,
        linux+etnaviv@...linux.org.uk, christian.gmeiner@...il.com,
        yuq825@...il.com, eric@...olt.net, thellstrom@...are.com,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        etnaviv@...ts.freedesktop.org, lima@...ts.freedesktop.org
Subject: Re: [PATCH 3/6] drm/gem: use new ww_mutex_(un)lock_for_each macros

Am 14.06.19 um 14:59 schrieb Peter Zijlstra:
> On Fri, Jun 14, 2019 at 02:41:22PM +0200, Christian König wrote:
>> Use the provided macros instead of implementing deadlock handling on our own.
>>
>> Signed-off-by: Christian König <christian.koenig@....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?

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);

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