linux-kernel - Re: [PATCH] drm/syncobj: make lockdep complain on WAIT_FOR

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Date:   Tue, 19 Jan 2021 10:35:38 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christian König 
        <ckoenig.leichtzumerken@...il.com>
Cc:     daniel@...ll.ch, mingo@...hat.com, will@...nel.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/syncobj: make lockdep complain on WAIT_FOR_SUBMIT v2

On Mon, Jan 18, 2021 at 07:03:34PM +0100, Christian König wrote:

> diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
> index 6e74e6745eca..f51458615158 100644
> --- a/drivers/gpu/drm/drm_syncobj.c
> +++ b/drivers/gpu/drm/drm_syncobj.c
> @@ -387,6 +387,13 @@ int drm_syncobj_find_fence(struct drm_file *file_private,
>  	if (!syncobj)
>  		return -ENOENT;
>  
> +	/* Waiting for userspace with locks help is illegal cause that can
> +	 * trivial deadlock with page faults for example. Make lockdep complain
> +	 * about it early on.
> +	 */

Egads, the cursed comment style is spreading :/

> +	if (flags & DRM_SYNCOBJ_WAIT_FLAGS_WAIT_FOR_SUBMIT)
> +		lockdep_assert_none_held_once();
> +

Should this not be part of drm_syncobj_fence_add_wait() instead? Also,
do you want to sprinkle might_sleep() around ?

>  	*fence = drm_syncobj_fence_get(syncobj);
>  	drm_syncobj_put(syncobj);
>  
> diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h