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SubjectRe: [PATCH 32/33] fbcon: Document what I learned about fbcon locking
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Op 20-05-2019 om 10:22 schreef Daniel Vetter:
> It's not pretty.
>
> Signed-off-by: Daniel Vetter <daniel.vetter@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>
> ---
> 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 b40b56702c61..cbbcf7a795f2 100644
> --- a/drivers/video/fbdev/core/fbcon.c
> +++ b/drivers/video/fbdev/core/fbcon.c
> @@ -87,6 +87,25 @@
> # 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 */

I did a casual review, so for whole series with the small nitpicks I had, and any feedback by others, kbuild and the arm mess being fixed up:

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

However, according to reviewer's statement of oversight:

While I have reviewed the patch and believe it to be sound, I do not (unless explicitly stated elsewhere)
make any warranties or guarantees that it will achieve its stated purpose or function properly in any given situation.

:)

~Maarten

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