linux-kernel - Re: [PATCH 4/7] drm/bridge: lock the encoder chain in scoped for

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Message-ID: <20251002173331.3d06d7d3@booty>
Date: Thu, 2 Oct 2025 17:33:31 +0200
From: Luca Ceresoli <luca.ceresoli@...tlin.com>
To: kernel test robot <oliver.sang@...el.com>
Cc: <oe-lkp@...ts.linux.dev>, <lkp@...el.com>,
 <dri-devel@...ts.freedesktop.org>, Maarten Lankhorst
 <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>,
 Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Andrzej Hajda <andrzej.hajda@...el.com>,
 Neil Armstrong <neil.armstrong@...aro.org>, Robert Foss <rfoss@...nel.org>,
 Laurent Pinchart <Laurent.pinchart@...asonboard.com>, Jonas Karlman
 <jonas@...boo.se>, Jernej Skrabec <jernej.skrabec@...il.com>, Hui Pu
 <Hui.Pu@...ealthcare.com>, Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/7] drm/bridge: lock the encoder chain in scoped
 for_each loops

Hello,

On Tue, 30 Sep 2025 14:16:23 +0800
kernel test robot <oliver.sang@...el.com> wrote:

> [  674.477504][    C1] watchdog: BUG: soft lockup - CPU#1 stuck for 626s! [swapper/0:1]
> [  674.477539][    C1] CPU#1 Utilization every 96s during lockup:
> [  674.477543][    C1] 	#1: 100% system,	  1% softirq,	  1% hardirq,	  0% idle
> [  674.477549][    C1] 	#2: 100% system,	  1% softirq,	  1% hardirq,	  0% idle
> [  674.477553][    C1] 	#3: 100% system,	  0% softirq,	  1% hardirq,	  0% idle
> [  674.477557][    C1] 	#4: 100% system,	  1% softirq,	  1% hardirq,	  0% idle
> [  674.477561][    C1] 	#5: 100% system,	  1% softirq,	  1% hardirq,	  0% idle
...
> [ 674.477713][ C1] drm_atomic_add_encoder_bridges (drivers/gpu/drm/drm_atomic.c:1319 (discriminator 1025)) 

The one reported is an actual bug, causing an infinite loop when the
encoder bridge chain is empty. Took a while to reproduce and debug, but
the fix is very simple:

 #define drm_for_each_bridge_in_chain_scoped(encoder, bridge)                           \
        for (struct drm_bridge *bridge __free(drm_bridge_encoder_chain_unlock) =        \
-                    list_first_entry(&drm_encoder_chain_lock(encoder)->bridge_chain,   \
+                    list_first_entry_or_null(&drm_encoder_chain_lock(encoder)->bridge_chain,   \

Fix queued for v2.

Luca

-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com