linux-kernel - Re: [PATCH 3/6] drm/gem: use new ww_mutex_(un)lock_for

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Date:   Fri, 14 Jun 2019 15:19:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christian König 
        <ckoenig.leichtzumerken@...il.com>
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

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);
>  
> -- 
> 2.17.1
>