linux-kernel - Re: [PATCH] drm/vkms: fix misuse of WARN

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Date:   Wed, 24 Mar 2021 19:00:08 -0300
From:   Melissa Wen <melissa.srw@...il.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     daniel@...ll.ch, airlied@...ux.ie, hamohammed.sa@...il.com,
        rodrigosiqueiramelo@...il.com,
        syzbot+4fc21a003c8332eb0bdd@...kaller.appspotmail.com,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/vkms: fix misuse of WARN_ON

On 03/20, Dmitry Vyukov wrote:
> vkms_vblank_simulate() uses WARN_ON for timing-dependent condition
> (timer overrun). This is a mis-use of WARN_ON, WARN_ON must be used
> to denote kernel bugs. Use pr_warn() instead.
> 
> Signed-off-by: Dmitry Vyukov <dvyukov@...gle.com>
> Reported-by: syzbot+4fc21a003c8332eb0bdd@...kaller.appspotmail.com
> Cc: Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>
> Cc: Melissa Wen <melissa.srw@...il.com>
> Cc: Haneen Mohammed <hamohammed.sa@...il.com>
> Cc: Daniel Vetter <daniel@...ll.ch>
> Cc: David Airlie <airlied@...ux.ie>
> Cc: dri-devel@...ts.freedesktop.org
> Cc: linux-kernel@...r.kernel.org
> Change-Id: I7f01c288092bc7e472ec63af198f93ce3d8c49f7
> ---
>  drivers/gpu/drm/vkms/vkms_crtc.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/vkms/vkms_crtc.c b/drivers/gpu/drm/vkms/vkms_crtc.c
> index 0443b7deeaef6..758d8a98d96b3 100644
> --- a/drivers/gpu/drm/vkms/vkms_crtc.c
> +++ b/drivers/gpu/drm/vkms/vkms_crtc.c
> @@ -18,7 +18,8 @@ static enum hrtimer_restart vkms_vblank_simulate(struct hrtimer *timer)
>  
>  	ret_overrun = hrtimer_forward_now(&output->vblank_hrtimer,
>  					  output->period_ns);
> -	WARN_ON(ret_overrun != 1);
> +	if (ret_overrun != 1)
> +		pr_warn("%s: vblank timer overrun\n", __func__);

Hi Dmitry,

Thanks for your patch.

Looks good to me.
The Change-Id tag just seems a little noisy to me, but can be
fixed while applying.

Acked-by: Melissa Wen <melissa.srw@...il.com>

>  
>  	spin_lock(&output->lock);
>  	ret = drm_crtc_handle_vblank(crtc);
> 
> base-commit: e94c55b8e0a0bbe9a026250cf31e2fa45957d776
> -- 
> 2.31.0.291.g576ba9dcdaf-goog
>