Re: [PATCH v2] fbcon: Avoid cancel_delayed_work_sync() in atomic context

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] fbcon: race in fbcon_del_cursor_work() lets fb_flashcursor() re-arm work
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commit e9056993ba6eca270987e2b77d6a3977beffec71
Author: Yao Yiqi <yaoyiqi3@huawei.com>

fbcon: Avoid cancel_delayed_work_sync() in atomic context

This commit replaces cancel_delayed_work_sync() with cancel_delayed_work() in
fbcon_del_cursor_work() to avoid sleeping in atomic console printing contexts,
while adding fbcon_del_cursor_work_sync() for teardown in fbcon_release() and
moving the delayed work requeue in fb_flashcursor() inside console_lock.

> diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
> index 7720dfc80c165..066cd974ea3bb 100644
> --- a/drivers/video/fbdev/core/fbcon.c
> +++ b/drivers/video/fbdev/core/fbcon.c
> @@ -422,10 +422,14 @@ static void fb_flashcursor(struct work_struct *work)
>  	par->bitops->cursor(vc, info, enable,
>  			    get_fg_color(vc, info, c),
>  			    get_bg_color(vc, info, c));
> -	console_unlock();
>  
> +	/*
> +	 * Re-arm under console_lock so that a concurrent
> +	 * fbcon_del_cursor_work() cannot race with this requeue.
> +	 */
>  	queue_delayed_work(system_power_efficient_wq, &par->cursor_work,
>  			   par->cur_blink_jiffies);
> +	console_unlock();
>  }
>  
>  static void fbcon_add_cursor_work(struct fb_info *info)
> @@ -437,10 +441,30 @@ static void fbcon_add_cursor_work(struct fb_info *info)
>  				   par->cur_blink_jiffies);
>  }
>  
> +/*
> + * Stop the cursor blink.
> + *
> + * Does not wait for a callback that is already running, so it is safe to
> + * call from any context.
> + *
> + * console_lock serializes this with the requeue in fb_flashcursor();
> + * otherwise a running callback could re-arm the work.
> + */
>  static void fbcon_del_cursor_work(struct fb_info *info)
>  {
>  	struct fbcon_par *par = info->fbcon_par;
>  
> +	cancel_delayed_work(&par->cursor_work);
> +}

[Severity: Medium]
Can fb_flashcursor() still race with fbcon_del_cursor_work() and re-arm
par->cursor_work after blinking has been stopped?

When par->cursor_work expires, process_one_work() clears
WORK_STRUCT_PENDING_BIT before invoking fb_flashcursor(). If a caller holding
console_lock (such as cursor_blink_store(), fbcon_blank(), fbcon_switch(),
fbcon_deinit(), or fbcon_cursor() with CUR_SW) runs fbcon_del_cursor_work()
after WORK_STRUCT_PENDING_BIT is cleared, but before the worker executes
console_trylock() in fb_flashcursor():

CPU0 (holding console_lock)          CPU1 (worker thread)
                                     process_one_work()
                                       /* clears WORK_STRUCT_PENDING_BIT */
                                       fb_flashcursor()
fbcon_del_cursor_work()
  cancel_delayed_work(&par->cursor_work)
  /* returns false without waiting */
console_unlock()
                                         ret = console_trylock();
                                         /* succeeds */

Because fb_flashcursor() only checks:

	if (!vc || !con_is_visible(vc) ||
	    fbcon_info_from_console(vc->vc_num) != info ||
	    vc->vc_deccm != 1) {
		console_unlock();
		return;
	}

and does not verify whether blinking is still supposed to be active (for
example fbcon_cursor_blink, vc->vc_cursor_type & CUR_SW,par->graphics /
KD_GRAPHICS, par->blank_state, or par->initialized), won't it draw the cursor
and call queue_delayed_work() to perpetually re-arm par->cursor_work?

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Sashiko AI review · https://sashiko.dev/#/patchset/20261027033422.585328-1-yyaqia@gmail.com?part=1