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Subject: Re: [PATCH] drm/syncobj: make lockdep complain on WAIT_FOR_SUBMIT v2
To:     =?UTF-8?Q?Christian_K=c3=b6nig?= <ckoenig.leichtzumerken@gmail.com>,
        daniel@ffwll.ch, peterz@infradead.org, mingo@redhat.com,
        will@kernel.org, dri-devel@lists.freedesktop.org,
        linux-kernel@vger.kernel.org
References: <20210118180334.43714-1-christian.koenig@amd.com>
 <20210118180334.43714-2-christian.koenig@amd.com>
From:   Randy Dunlap <rdunlap@infradead.org>
Message-ID: <3ebc7345-1b3f-8e7f-dc12-eacfd3a993d9@infradead.org>
Date:   Mon, 18 Jan 2021 11:42:06 -0800
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Hi,

Just a comment about the comments:

On 1/18/21 10:03 AM, Christian König wrote:
> DRM_SYNCOBJ_WAIT_FLAGS_WAIT_FOR_SUBMIT can't be used when we hold locks
> since we are basically waiting for userspace to do something.
> 
> Holding a lock while doing so can trivial deadlock with page faults
> etc...
> 
> So make lockdep complain when a driver tries to do this.
> 
> v2: Add lockdep_assert_none_held() macro.
> 
> Signed-off-by: Christian König <christian.koenig@amd.com>
> ---
>  drivers/gpu/drm/drm_syncobj.c | 7 +++++++
>  include/linux/lockdep.h       | 5 +++++
>  2 files changed, 12 insertions(+)
> 
> 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

	                                    held            because

> +	 * trivial deadlock with page faults for example. Make lockdep complain

	   trivially

> +	 * about it early on.
> +	 */
> +	if (flags & DRM_SYNCOBJ_WAIT_FLAGS_WAIT_FOR_SUBMIT)
> +		lockdep_assert_none_held_once();
> +
>  	*fence = drm_syncobj_fence_get(syncobj);
>  	drm_syncobj_put(syncobj);
>  


thanks.
-- 
~Randy
You can't do anything without having to do something else first.
-- Belefant's Law