linux-kernel - [PATCH 31/33] fbcon: Document what I learned about fbcon locking

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Message-Id: <20190524085354.27411-32-daniel.vetter@ffwll.ch>
Date:   Fri, 24 May 2019 10:53:52 +0200
From:   Daniel Vetter <daniel.vetter@...ll.ch>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Yisheng Xie <ysxie@...mail.com>
Subject: [PATCH 31/33] fbcon: Document what I learned about fbcon locking

It's not pretty.

Signed-off-by: Daniel Vetter <daniel.vetter@...el.com>
Cc: Daniel Vetter <daniel.vetter@...ll.ch>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Cc: Hans de Goede <hdegoede@...hat.com>
Cc: Yisheng Xie <ysxie@...mail.com>
---
 drivers/video/fbdev/core/fbcon.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index 6a4bbb8407c0..8444d5151c2d 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -88,6 +88,25 @@
 #  define DPRINTK(fmt, args...)
 #endif
 
+/*
+ * FIXME: Locking
+ *
+ * - fbcon state itself is protected by the console_lock, and the code does a
+ *   pretty good job at making sure that lock is held everywhere it's needed.
+ *
+ * - access to the registered_fb array is entirely unprotected. This should use
+ *   proper object lifetime handling, i.e. get/put_fb_info. This also means
+ *   switching from indices to proper pointers for fb_info everywhere.
+ *
+ * - fbcon doesn't bother with fb_lock/unlock at all. This is buggy, since it
+ *   means concurrent access to the same fbdev from both fbcon and userspace
+ *   will blow up. To fix this all fbcon calls from fbmem.c need to be moved out
+ *   of fb_lock/unlock protected sections, since otherwise we'll recurse and
+ *   deadlock eventually. Aside: Due to these deadlock issues the fbdev code in
+ *   fbmem.c cannot use locking asserts, and there's lots of callers which get
+ *   the rules wrong, e.g. fbsysfs.c entirely missed fb_lock/unlock calls too.
+ */
+
 enum {
 	FBCON_LOGO_CANSHOW	= -1,	/* the logo can be shown */
 	FBCON_LOGO_DRAW		= -2,	/* draw the logo to a console */
-- 
2.20.1