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fbcon: Document what I learned about fbcon locking

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It's not pretty.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Yisheng Xie <ysxie@foxmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190528090304.9388-32-daniel.vetter@ffwll.ch
This commit is contained in:
Daniel Vetter 2019-05-28 11:03:02 +02:00
parent fe2d70d6f6
commit 94193d2aae
1 changed files with 19 additions and 0 deletions
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19
drivers/video/fbdev/core/fbcon.c
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@ -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 */