Re: [PATCH] fbcon: use default if cursor blink interval is not valid

David Daney
2016-05-19T15:59:12Z

On 05/18/2016 09:21 PM, Scot Doyle wrote:
> Two current [1] and three previous [2] systems locked during boot
> because the cursor flash timer was set using an ops->cur_blink_jiffies
> value of 0. Previous patches attempted to solve the problem by moving
> variable initialization earlier in the setup sequence [2].
>
> Use the normal cursor blink default interval of 200 ms if
> ops->cur_blink_jiffies is not in the range specified in commit
> bd63364caa8d. Since invalid values are not used, specific system
> initialization timings should not cause lockups.
>

This patch just papers over the problem that you yourself introduced in
commit bd63364caa8d ("vt: add cursor blink interval escape sequence").

As you know, I have a patch that fixes the problem at the source:
https://lkml.org/lkml/2016/5/17/455

I don't like the idea of silently ignoring bad values passed in from
other code (drivers/tty/vt/vt.c), and much less doing the check for bad
values each time the timer expires rather than just once, where the bad
value is first introduced.

I think it would be preferable to WARN() at the site the bad value is
introduced, so that we can easily find the real source of the problem.
Initialize cur_blink_jiffies to a sane default value, then if something
attempts to set it to a value that would cause soft lockup, WARN and
refuse to change it.

Also there is a stylistic issue...

> [1] https://bugs.launchpad.net/bugs/1574814
> [2] see commits: 2a17d7e80f1d, f235f664a8af, a1e533ec07d5
>
> Signed-off-by: Scot Doyle <lkml14@scotdoyle.com>
> Cc: <stable@vger.kernel.org> [v4.2]
> ---
> drivers/video/console/fbcon.c | 17 +++++++++++++----
> 1 file changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/video/console/fbcon.c b/drivers/video/console/fbcon.c
> index 6e92917..da61d87 100644
> --- a/drivers/video/console/fbcon.c
> +++ b/drivers/video/console/fbcon.c
> @@ -396,13 +396,23 @@ static void fb_flashcursor(struct work_struct *work)
> console_unlock();
> }
>
> +static int cursor_blink_jiffies(int candidate)
> +{
> + if (candidate >= msecs_to_jiffies(50) &&
> + candidate <= msecs_to_jiffies(USHRT_MAX))
> + return candidate;
> + else
> + return HZ / 5;

You use msecs_to_jiffies() is several places, then here open code the
division. Please use msecs_to_jiffies(), that is it's intended job.

> +}
> +
> static void cursor_timer_handler(unsigned long dev_addr)
> {
> struct fb_info *info = (struct fb_info *) dev_addr;
> struct fbcon_ops *ops = info->fbcon_par;
>
> queue_work(system_power_efficient_wq, &info->queue);
> - mod_timer(&ops->cursor_timer, jiffies + ops->cur_blink_jiffies);
> + mod_timer(&ops->cursor_timer, jiffies +
> + cursor_blink_jiffies(ops->cur_blink_jiffies));
> }
>
> static void fbcon_add_cursor_timer(struct fb_info *info)
> @@ -417,7 +427,8 @@ static void fbcon_add_cursor_timer(struct fb_info *info)
>
> init_timer(&ops->cursor_timer);
> ops->cursor_timer.function = cursor_timer_handler;
> - ops->cursor_timer.expires = jiffies + ops->cur_blink_jiffies;
> + ops->cursor_timer.expires = jiffies +
> + cursor_blink_jiffies(ops->cur_blink_jiffies);
> ops->cursor_timer.data = (unsigned long ) info;
> add_timer(&ops->cursor_timer);
> ops->flags |= FBCON_FLAGS_CURSOR_TIMER;
> @@ -709,7 +720,6 @@ static int con2fb_acquire_newinfo(struct vc_data *vc, struct fb_info *info,
> }
>
> if (!err) {
> - ops->cur_blink_jiffies = HZ / 5;
> info->fbcon_par = ops;
>
> if (vc)
> @@ -957,7 +967,6 @@ static const char *fbcon_startup(void)
> ops->currcon = -1;
> ops->graphics = 1;
> ops->cur_rotate = -1;
> - ops->cur_blink_jiffies = HZ / 5;
> info->fbcon_par = ops;
> p->con_rotate = initial_rotation;
> set_blitting_type(vc, info);
>