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Date:   Fri, 14 Jun 2019 20:10:20 +0200
From:   Christian König <ckoenig.leichtzumerken@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>, 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 17:22 schrieb Daniel Vetter:
> On Fri, Jun 14, 2019 at 03:19:16PM +0200, Peter Zijlstra wrote:
>> 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;
>>> -			}
>>> -
>>> -			ww_acquire_done(acquire_ctx);
>>> -			return ret;
>>> -		}
>>> -	}
>> I note all the sites you use this on are simple idx iterators; so how
>> about something like so:
>>
>> int ww_mutex_unlock_all(int count, void *data, struct ww_mutex *(*func)(int, void *))
>> {
>> 	int i;
>>
>> 	for (i = 0; i < count; i++) {
>> 		lock = func(i, data);
>> 		ww_mutex_unlock(lock);
>> 	}
>> }
>>
>> int ww_mutex_lock_all(int count, struct ww_acquire_context *acquire_ctx, bool intr,
>> 		      void *data, struct ww_mutex *(*func)(int, void *))
>> {
>> 	int i, ret, contended = -1;
>> 	struct ww_mutex *lock;
>>
>> retry:
>> 	if (contended != -1) {
>> 		lock = func(contended, data);
>> 		if (intr)
>> 			ret = ww_mutex_lock_slow_interruptible(lock, acquire_ctx);
>> 		else
>> 			ret = ww_mutex_lock_slow(lock, acquire_ctx), 0;
>>
>> 		if (ret) {
>> 			ww_acquire_done(acquire_ctx);
>> 			return ret;
>> 		}
>> 	}
>>
>> 	for (i = 0; i < count; i++) {
>> 		if (i == contended)
>> 			continue;
>>
>> 		lock = func(i, data);
>> 		if (intr)
>> 			ret = ww_mutex_lock_interruptible(lock, acquire_ctx);
>> 		else
>> 			ret = ww_mutex_lock(lock, acquire_ctx), 0;
>>
>> 		if (ret) {
>> 			ww_mutex_unlock_all(i, data, func);
>> 			if (contended > i) {
>> 				lock = func(contended, data);
>> 				ww_mutex_unlock(lock);
>> 			}
>>
>> 			if (ret == -EDEADLK) {
>> 				contended = i;
>> 				goto retry;
>> 			}
>>
>> 			ww_acquire_done(acquire_ctx);
>> 			return ret;
>> 		}
>> 	}
>>
>> 	ww_acquire_done(acquire_ctx);
>> 	return 0;
>> }
>>
>>> +	ww_mutex_lock_for_each(for (i = 0; i < count; i++),
>>> +			       &objs[i]->resv->lock, contended, ret, true,
>>> +			       acquire_ctx)
>>> +		if (ret)
>>> +			goto error;
>> which then becomes:
>>
>> struct ww_mutex *gem_ww_mutex_func(int i, void *data)
>> {
>> 	struct drm_gem_object **objs = data;
>> 	return &objs[i]->resv->lock;
>> }
>>
>> 	ret = ww_mutex_lock_all(count, acquire_ctx, true, objs, gem_ww_mutex_func);
>>
>>>   	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);
> Another idea, entirely untested (I guess making sure that we can use the
> same iterator for both locking and unlocking in the contended case will be
> fun), but maybe something like this:
>
> 	WW_MUTEX_LOCK_BEGIN();
> 	driver_for_each_loop (iter, pos) {
> 		WW_MUTEX_LOCK(&pos->ww_mutex);
> 	}
> 	WW_MUTEX_LOCK_END();
>
> That way we can reuse any and all iterators that'll ever show up at least.
> It's still horrible because the macros need to jump around between all of
> them.

Yeah, I tried this as well and that's exactly the reason why I discarded 
this approach.

There is this hack with goto *void we could use, but I'm pretty sure 
that is actually not part of any C standard.

> Would also make this useful for more cases, where maybe you need to
> trylock some lru lock to get at your next ww_mutex, or do some
> kref_get_unless_zero. Buffer eviction loops tend to acquire these, and
> that would all get ugly real fast if we'd need to stuff it into some
> iterator argument.

Well I don't see a use case with eviction in general. The dance there 
requires something different as far as I can see.

Christian.

> This is kinda what we went with for modeset locks with
> DRM_MODESET_LOCK_ALL_BEGIN/END, you can grab more locks in between the
> pair at least. But it's a lot more limited use-cases, maybe too fragile an
> idea for ww_mutex in full generality.
>
> Not going to type this out because too much w/e mode here already, but I
> can give it a stab next week.
> -Daniel

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